How to Simulate the Feeling of Head: A Comprehensive Guide to Sensory Immersion

Understanding the Nuance: How to Simulate the Feeling of Head

The quest to accurately simulate the feeling of "head" isn't merely about mimicking a physical sensation; it delves into the complex interplay of proprioception, vestibular input, and even emotional resonance. When we talk about the "feeling of head," we're referring to that innate sense of our head's position in space, its weight, its subtle movements, and the way it interacts with our body and environment. This can be crucial in various fields, from virtual reality and gaming to rehabilitation and artistic expression. For instance, I recall a VR experience designed to simulate a first-person perspective during a roller coaster ride. While the visuals and audio were incredibly immersive, the lack of any believable "head" sensation left a disconnect, making the experience feel less like *being there* and more like *watching something*. This is where the challenge of simulating the feeling of head truly arises – bridging that gap between visual perception and embodied reality.

The Foundation: What Constitutes the "Feeling of Head"?

Before we can even begin to simulate it, we must first dissect what constitutes the feeling of head. It's not a single sensation but a confluence of sensory inputs. Proprioception: This is our body's internal sense of its own position and movement. Within the head, this involves the muscles and joints in our neck, skull, and jaw, providing feedback about our head's orientation and the effort required to move it. Vestibular System: Located in the inner ear, the vestibular system is our primary organ for balance and spatial orientation. It detects linear acceleration (like moving forward or backward) and angular acceleration (like tilting your head or turning). This system is absolutely critical for that dizzying or stable sensation we associate with head movements. Cutaneous Sensation: The feeling of touch on our scalp, face, and ears contributes to our awareness of our head. This can range from the gentle brush of hair to the pressure of a hat or the sting of cold air. Cervical Spine Feedback: The complex network of nerves and muscles in the cervical spine provides constant, albeit often subconscious, information about our head's posture and any strain or tension. Interoception: This is the sense of the internal state of our body. While less direct, internal sensations like a headache, or even the feeling of blood rushing to the head, can profoundly influence our perception of our head. Visual-Vestibular Integration: Our brain constantly combines visual information with vestibular input. When we move our head, our eyes also move to maintain a stable field of vision. This integration is key to feeling grounded and oriented. My personal exploration into this topic began when I was developing a motion capture system for animation. We were focused on capturing the nuances of facial expressions, but the animators kept lamenting the lack of believable head movement that conveyed personality and intent. It wasn't just about rotating the head; it was about how the neck muscles engaged, the subtle shifts in weight, and the overall *presence* of the head. This led me down a rabbit hole of understanding how we, as humans, intrinsically know what it feels like to have a head.

Simulating Head Movement: The Mechanics of Motion

Perhaps the most direct way to simulate the feeling of head is by replicating its movement. This involves understanding the degrees of freedom and the subtle forces at play. Degrees of Freedom in Head Movement The human head can move in several ways, each contributing to the overall sensation: Flexion/Extension: Nodding your head up and down. Lateral Flexion: Tilting your head to the side. Rotation: Turning your head left or right. Protraction/Retraction: Jutting your head forward or pulling it back. Each of these movements is driven by a complex interplay of muscles, ligaments, and bony structures. When we simulate these movements, we need to consider not just the *direction* but also the *resistance*, *inertia*, and *acceleration*. Mechanical Simulation Approaches There are several ways to mechanically simulate head movement: Robotic Actuators: In high-end simulation systems, robotic arms or specialized actuators can be used to physically move a user's head or a helmet. These systems can be programmed to mimic specific movements with a high degree of accuracy, providing a tangible sense of motion. However, these are often bulky, expensive, and can pose safety concerns if not meticulously designed. Force Feedback Devices: These devices provide resistance or force to a user's head movements. Imagine a system where tilting your head forward encounters a gentle but firm resistance, much like moving your head against gravity. This can create a more convincing sensation of effort and weight. Haptic Suits and Exoskeletons: While not directly simulating head movement in the traditional sense, haptic feedback integrated into head-mounted displays or neck-worn devices can provide tactile cues that enhance the feeling of head orientation. For example, a gentle vibration when your virtual head bumps into an object. From my experience, simply rotating a camera in a virtual environment doesn't replicate the feeling. You need to introduce forces and resistances that your body instinctively expects. For instance, when you tilt your head down, there's a subtle pull due to gravity. Replicating this gentle downward pull, even through haptic feedback, can make a significant difference in perceived realism.

Simulating Vestibular Input: The Illusion of Motion

The vestibular system is the unsung hero of spatial awareness. Simulating its input is key to convincing users they are experiencing motion. Challenges in Vestibular Simulation Directly stimulating the vestibular system is incredibly difficult and often leads to motion sickness. The goal, therefore, is to *trick* the brain into believing motion is occurring by providing congruent sensory information. Indirect Simulation Techniques Visual Cues: This is the most common and accessible method. When your virtual head moves, the visual scene must react in perfect synchrony. This includes the apparent motion of objects in the environment (parallax), head-bobbing effects that mimic natural walking, and the way peripheral vision blurs during rapid movement. Audio Cues: The sound of wind rushing past, the creak of a moving vehicle, or the subtle sounds associated with specific head movements (like a faint "whoosh" when turning quickly) can significantly enhance the feeling of motion. Motion Platforms: In conjunction with visual and audio, motion platforms that tilt and move the entire user base can provide a more robust sense of physical motion, complementing the simulated head movements. Controlled Acceleration and Deceleration: The way visual elements accelerate and decelerate in a simulated environment can mimic the sensation of linear and angular acceleration that the vestibular system detects. This is often achieved through subtle camera movements and field-of-view adjustments. I remember working on a flight simulator where the primary feedback was visual and audio. When the pilot performed a sharp turn, the visuals would bank dramatically, and the sound of the engines would change. However, without any complementary motion on the platform, it still felt somewhat detached. It was only when we incorporated a more dynamic motion platform that the feeling of "banking" – and thus, the feeling of the head reacting to that bank – truly began to emerge. This highlights the critical need for multi-sensory congruence.

Haptic Feedback and Tactile Sensation: Feeling the Head

Beyond just movement, the tactile sensations associated with our head play a role in our embodied experience. Types of Tactile Feedback for the Head Pressure: Simulating the pressure of a helmet, a tight cap, or even the sensation of air pushing against your face. This can be achieved through inflatable bladders or precisely controlled actuators. Vibration: Subtle vibrations can mimic the feeling of external forces, such as being in a noisy environment or experiencing a slight impact. Temperature: While less common, simulating changes in temperature – like a cool breeze on the face – can enhance immersion. Airflow: Fans or directed air jets can simulate wind or the sensation of moving at speed. Consider the experience of wearing a VR headset. The physical pressure of the headset itself is a constant tactile input. If a simulation can augment this by, say, simulating the sensation of wind resistance on your face when you're moving fast in a virtual car, it adds another layer of believable "head" sensation. I've seen prototypes that use small fans integrated into VR masks to achieve this, and the effect, while simple, is surprisingly effective.

Simulating Weight and Inertia: The Mass of the Head

Our head isn't weightless; it has mass. This mass contributes to how we perceive its movement and how it affects our balance. Understanding the Physics of Head Weight The average adult human head weighs around 10 to 11 pounds. This weight is significant and is constantly being acted upon by gravity. When we move our head, our neck muscles work against this weight and inertia. Simulating Weight and Inertia Gravity Compensation: In virtual environments, if you're looking downwards, there should be a subtle feeling of your head being pulled down. This can be simulated through haptic feedback or by subtly adjusting the virtual camera's position to account for gravity. Inertial Dampening/Resistance: When you make a sudden head movement, your head naturally resists that change in motion due to inertia. Simulating this resistance, perhaps with force feedback, can make movements feel more natural. Conversely, in high-speed scenarios, you might want to simulate inertial effects that push your head in a certain direction. Center of Mass Awareness: The brain is acutely aware of the head's center of mass. When the head moves, especially rapidly, the brain anticipates the shift in the body's overall center of mass and makes subtle postural adjustments. Simulating the *consequences* of head movement on the body's balance can enhance the feeling. For a simulation to feel truly convincing, it needs to acknowledge the physical properties of the head. When I simulate a user turning their head quickly in a game, I don't just want the camera to snap. I want to feel a slight lag, a resistance, as if my neck muscles are actually doing the work. This is where understanding inertia becomes paramount.

The Role of Proprioception and Kinesthesia: Internal Sensations

Proprioception and kinesthesia are our internal senses of bodily position and movement. Simulating these is about creating the *feeling* of the head's state, not just its external motion. Enhancing Proprioceptive Feedback Neck Muscle Engagement Simulation: Through subtle vibrations or resistance in a neckband or haptic suit, one could potentially simulate the feeling of neck muscles tensing or relaxing during head movements. Subtle Head Bobbing: Natural walking involves a slight up-and-down and side-to-side motion of the head. Replicating this subtle, rhythmic motion in VR can significantly enhance the sense of embodiment and head position. Postural Cues: If the virtual avatar's head posture changes (e.g., slumping forward when tired), subtle haptic cues could be used to suggest this change in internal posture. This is where the nuance really comes in. It's not just about *seeing* your head move; it's about *feeling* your neck muscles engage. While replicating this precisely is challenging, even minor cues, like a slight vibration pattern that corresponds to assumed neck muscle tension during a sharp turn, can contribute to a more grounded sensation.

Emotional and Cognitive Aspects: The Subjective Experience

The feeling of head is also influenced by our emotional and cognitive state. How Emotions Affect Head Sensation Stress and Tension: When stressed, we often experience tension in our neck and shoulders, which directly impacts our head's perceived posture and comfort. Excitement and Adrenaline: A rush of adrenaline can lead to a feeling of blood rushing to the head, or a heightened sense of awareness. Fatigue: When tired, our head might feel heavier, and our posture might change, leading to a different subjective experience. For example, in a horror game, the simulated feeling of your head drooping or becoming heavy when you're scared could be a powerful emotional cue. This goes beyond simple physical simulation and taps into how our internal states color our perception of our physical self. Cognitive Load and Perception The brain's processing power influences how we perceive sensory input. In highly demanding virtual environments, the cognitive load can affect how much attention is paid to subtle head sensations.

Practical Applications of Simulating the Feeling of Head

The ability to simulate the feeling of head has far-reaching applications across various industries. Virtual Reality and Gaming Enhanced Immersion: This is the most obvious application. A more believable head sensation makes virtual worlds feel more real, leading to deeper immersion and presence. Improved Locomotion: Simulating natural head bobbing and sway during virtual locomotion can reduce motion sickness and make movement feel more intuitive. More Engaging Experiences: From fighting games where head movements can be used for dodging to narrative experiences where a character's head posture conveys emotion, a simulated head adds a new dimension. I've always believed that VR's true potential lies in its ability to trick our senses into accepting a new reality. And the feeling of having a head, with all its associated sensations, is fundamental to that acceptance. Without it, you're just a disembodied camera. Medical and Rehabilitation Vestibular Rehabilitation: Simulating specific head movements and their associated sensory feedback can be used in therapy for individuals with balance disorders or vertigo. Neurological Assessment: Devices that can accurately measure and replicate head movements could be valuable tools for assessing neurological conditions affecting motor control and spatial awareness. Pain Management: For individuals experiencing chronic neck pain, simulations could potentially be used to explore safe movement patterns or to provide distraction. Imagine a physical therapist using a system that guides a patient through a specific set of head movements, providing subtle haptic feedback to ensure correct form and minimizing discomfort. This could revolutionize how certain vestibular and neck-related conditions are treated. Robotics and Human-Computer Interaction Teleoperation: When a human operator controls a robot remotely, simulating the head's position and movement can provide a more intuitive and immersive control experience. Humanoid Robot Design: Understanding how humans perceive head movement can inform the design of more natural and relatable humanoid robots. Ergonomic Design: Simulating head and neck strain during various tasks can help in designing more ergonomic workstations and equipment. When controlling a robotic arm with a head-mounted display, if the robot's camera mirrors your head movements accurately, and you can even feel subtle forces transmitted back, it makes operating that robot feel far more direct and less like manipulating a tool. Artistic and Expressive Applications Performance Art: Artists could use wearable technology to translate their head movements and emotions into visual or auditory art in real-time. Character Animation: For animators, understanding the nuances of head simulation can lead to more lifelike and emotionally resonant character performances. I've seen artists use motion capture to create abstract visual pieces that react to their body movements. Imagine an artist's head movements directly influencing a symphony of lights or a fluid digital sculpture. This is where the expressive potential truly shines.

Building a Simulator: Key Components and Considerations

Creating a system that effectively simulates the feeling of head requires a multi-faceted approach. Hardware Components Head-Mounted Display (HMD): Essential for visual input, the quality of the display (resolution, refresh rate, field of view) significantly impacts immersion. Motion Tracking Sensors: Inertial Measurement Units (IMUs), optical trackers, or other sensors to accurately capture head orientation and movement in real-time. Haptic Feedback Devices: This is where the simulation of "feeling" comes in. Options include: Neckbands/Collars: With integrated actuators for vibration, pressure, or gentle force feedback. Face/Scalp Tactile Arrays: For simulating airflow, pressure, or even subtle temperature changes. Motion Platforms: For larger-scale, full-body motion that complements head movement. Audio Systems: High-quality headphones for spatial audio, which is crucial for anchoring head movements in a perceived environment. The integration of these components is key. A lag between physical head movement and visual or haptic response can quickly break the illusion and induce nausea. Software and Algorithms Real-time Tracking and Rendering: Algorithms to process sensor data and update the virtual environment and haptic feedback instantaneously. Physics Engines: To accurately simulate forces like gravity and inertia acting on the virtual head. Sensory Integration Algorithms: To synchronize visual, audio, and haptic cues for maximum believability. This often involves sophisticated techniques to smooth out jerky movements and minimize latency. User Personalization: Algorithms that can adapt the simulation to individual user sensitivities and preferences, particularly regarding motion sickness. The "secret sauce" often lies in the software. How do you take raw sensor data and translate it into a convincing sensory experience? This involves a lot of fine-tuning, often based on extensive user testing. User Experience Design Onboarding and Calibration: Ensuring users can easily set up and calibrate the system for optimal performance. Comfort and Ergonomics: The hardware must be comfortable to wear for extended periods. Reducing Motion Sickness: Implementing techniques like visual vignetting (darkening the periphery during movement), maintaining stable horizons, and providing clear locomotion options. Feedback Granularity: The level of detail in haptic feedback needs to be carefully considered. Too much can be distracting; too little can be unconvincing. My most frustrating experiences in VR have always been those where the system felt "off." A slight disconnect between my head turn and the visual response, or an unexpected jolt from a haptic device. Good UX design is about anticipating and mitigating these potential pitfalls.

Challenges and Future Directions

While significant progress has been made, simulating the feeling of head remains a complex challenge. Current Limitations Fidelity of Haptic Feedback: Replicating the subtle nuances of muscle tension, pressure, and kinesthetic awareness is incredibly difficult with current haptic technology. Motion Sickness: Despite advancements, the sensory mismatch that causes motion sickness remains a significant hurdle, especially with more dynamic head movements. Cost and Accessibility: High-fidelity simulation systems are often prohibitively expensive, limiting their widespread adoption. Individual Variability: People's sensory perception and susceptibility to motion sickness vary greatly, making a one-size-fits-all solution challenging. I often find myself comparing current haptic technology to early audio synthesizers. They can produce sounds, but they lack the richness and complexity of real instruments. We're getting there with haptics, but the "instrument" of the human head and neck is incredibly sophisticated. Emerging Technologies Advanced Haptic Actuators: Developments in electroactive polymers and pneumatic systems promise more nuanced and localized tactile feedback. Brain-Computer Interfaces (BCIs): While still in early stages for sensory simulation, BCIs could one day allow for more direct neural stimulation to create artificial sensations. AI-Driven Sensory Prediction: AI could potentially predict user intent and pre-emptively provide sensory feedback, reducing perceived latency. Biometric Integration: Using real-time biometric data (e.g., heart rate, muscle tension) to dynamically adjust the simulation and enhance emotional resonance. The future is exciting. Imagine a system that doesn't just react to your movements but anticipates them, subtly nudging your perception to make the experience feel even more natural. Or a system that can detect when you're feeling stressed and adjust the simulation accordingly.

Frequently Asked Questions about Simulating Head Sensation

Q1: How can I best simulate the feeling of head movement in my own VR setup at home? A1: For a home VR setup, the most accessible way to simulate the feeling of head movement primarily relies on visual and auditory cues, coupled with good tracking. First and foremost, ensure your VR headset has accurate, low-latency head tracking. This is the foundation. Beyond that, the visual design of your VR experience is paramount. Developers should implement realistic head-bobbing effects when simulating walking or running to mimic natural human locomotion. This subtle up-and-down and side-to-side motion provides a proprioceptive cue that your head is moving naturally. Field-of-view adjustments can also help; a slight narrowing of the field of view during rapid turns can trick the brain into perceiving faster movement and more inertia. Complementing the visuals, spatial audio is incredibly important. The sound of wind rushing past, or the acoustics of a room changing as you turn your head, can significantly enhance the sensation of spatial awareness and head orientation. For instance, if you're in a virtual forest, the rustling of leaves should change dynamically as you turn your head, making it feel more immersive. While advanced haptic feedback for the head is still quite specialized and expensive, you can experiment with subtle forms. Some VR controllers can provide localized vibrations. If the VR experience allows, mapping these vibrations to head impacts or directional forces can add a rudimentary tactile layer. For instance, if your virtual character bumps their head, a vibration in the controller held near your head (if applicable) might provide a slight cue. More sophisticated home setups might consider neckbands with small vibration motors, though these are less common and require specific software integration. Ultimately, a combination of precise tracking, realistic visual motion, convincing spatial audio, and thoughtful game design will get you closest to simulating the feeling of head movement at home. Q2: Why is it so difficult to accurately simulate the feeling of head weight and inertia? A2: Accurately simulating the feeling of head weight and inertia is challenging primarily because these sensations are deeply rooted in our body's proprioceptive and vestibular systems, and replicating these complex biological signals artificially is incredibly difficult. Our brains are finely tuned to the subtle interplay of gravity, muscle engagement, and the body's resistance to changes in motion (inertia). When you tilt your head down, your neck muscles actively work against gravity, and your vestibular system registers the angular acceleration. The feeling of weight isn't just a static pull; it's the dynamic effort your body exerts to maintain posture and control. Simulating this requires sophisticated haptic feedback systems that can precisely apply forces and resistances that mimic these biological responses. Current haptic technology, while advancing, often relies on vibrations, localized pressure, or basic force feedback. To truly simulate head weight, you would need a system capable of applying nuanced, dynamic forces that change based on orientation and acceleration, much like the muscles and ligaments in your neck do. Inertia, the resistance to acceleration, is similarly complex. When you whip your head around, the momentum of your head naturally pulls on your neck. Replicating this feeling of being "pulled" or "resisted" requires more than just visual cues; it needs a physical force applied to the head or neck. Furthermore, the brain integrates these sensations seamlessly with visual and vestibular input. A mismatch, such as seeing your head move but not feeling the corresponding pull or resistance, can be jarring and even lead to motion sickness. Therefore, any artificial simulation must not only replicate the physical forces but also do so in perfect synchrony with other sensory inputs. The cost and complexity of developing such a system, capable of applying precise and dynamic forces to the head and neck in real-time, are significant barriers, making it a difficult aspect to accurately simulate in most consumer or even professional applications. Q3: What are the primary sensory inputs that contribute to our perception of having a "head" in space? A3: Our perception of having a "head" in space is a rich tapestry woven from multiple sensory inputs, working in concert to create a strong sense of embodiment. At the forefront is our **vestibular system**, located in the inner ear. This system is exquisitely sensitive to gravity, linear acceleration (like moving forward in a car), and angular acceleration (like tilting your head). It continuously informs the brain about the head's orientation relative to gravity and its movements, providing that fundamental sense of being upright or in motion. Equally crucial is **proprioception**, the body's internal sense of its own position and movement. Within the context of the head, this involves signals from the muscles, tendons, and joints in our neck and even our jaw. These receptors tell the brain about the angle of our neck, the effort being exerted by our muscles to hold our head up or move it, and any tension present. This provides a detailed, internal map of our head's posture and dynamics. **Cutaneous sensation**, the sense of touch and pressure on the skin, also plays a role. Feeling the air on our face, the weight of our hair, or the pressure of a hat or helmet contributes to our awareness of our head's physical presence and boundaries. Even subtle changes in temperature on the face can reinforce this sense. The **cervical spine** itself provides a wealth of sensory information. Nerves in the neck are highly attuned to posture and movement, feeding constant, often subconscious, data to the brain about the head's position relative to the torso. Finally, **visual-vestibular integration** is key. Our brain constantly correlates what our eyes see with what our vestibular system senses. The way the world appears to move or stabilize as we turn our head is a critical component of our spatial awareness and our feeling of being grounded in an environment. Collectively, these sensory streams create our innate, usually unconscious, perception of having a head that is situated and oriented within the world. Q4: How can simulating the feeling of head movement help reduce motion sickness in virtual reality? A4: Simulating the feeling of head movement, particularly through realistic visual and even subtle haptic cues, can significantly help reduce motion sickness in virtual reality by aligning sensory inputs and reducing the conflict between what the eyes see and what the inner ear (vestibular system) feels. Motion sickness often arises from a sensory mismatch: the eyes perceive movement within the VR environment, but the inner ear, not physically moving, signals that the body is stationary. This discrepancy confuses the brain, leading to feelings of nausea. When the simulation accurately reflects natural head movements, it helps bridge this gap. For example, implementing realistic head-bobbing during virtual locomotion (walking or running) mimics the subtle, rhythmic motion that our heads naturally make during terrestrial movement. This provides a visual and sometimes proprioceptive cue that aligns with what the body *would* be feeling if it were actually moving. Similarly, if a virtual vehicle turns sharply, accurately simulating the visual banking and potentially adding a subtle sensation of inertia or resistance (even through gentle haptic feedback or screen effects like vignetting) can make the visual motion feel more congruent with expected physical sensations. Furthermore, some advanced simulations aim to provide very subtle physical cues that mimic the head's reaction to motion. This could involve a gentle force feedback in a neck-worn device that simulates the pull of gravity when tilting the head, or a slight vibration pattern that suggests neck muscle engagement. These tactile inputs, when synchronized with visual motion, can provide an additional layer of sensory information that helps the brain reconcile the perceived movement. By providing more congruent and expected sensory feedback related to head movement, simulations can trick the brain into believing the body's sensory systems are in agreement, thereby mitigating the disorientation and nausea associated with sensory conflict. Q5: Are there specific types of VR experiences or games that benefit most from advanced head simulation? A5: Yes, certain types of VR experiences and games benefit disproportionately from advanced head simulation because they rely heavily on our innate sense of presence, spatial orientation, and physical interaction with the environment. * **Flight Simulators and Vehicle Simulators:** These are perhaps the most obvious beneficiaries. When piloting an aircraft, car, or spacecraft, the pilot's head movements are critical for situational awareness, scanning the environment, and reacting to G-forces. Advanced head simulation, which includes realistic visual cues of banking, acceleration, and the feeling of inertia, makes these experiences far more immersive and believable. The ability to "feel" the G-force through subtle haptics or motion platforms, synchronized with head movements, dramatically enhances the sense of being in control and experiencing the dynamics of flight or high-speed driving. * **First-Person Shooters (FPS) and Action Games:** In fast-paced action games, precise head movement is often used for aiming, dodging, and surveying the battlefield. A more realistic feeling of head movement, including accurate inertia and weight, can lead to more intuitive aiming and a stronger sense of embodiment. When a character ducks or dodges, simulating the physical effort and inertia involved can make these actions feel more impactful and responsive, enhancing player agency and immersion. * **Horror and Thriller Experiences:** These genres thrive on creating a visceral emotional response. Advanced head simulation can amplify fear and tension by making the virtual environment feel more tangible and threatening. For instance, a subtle simulation of the head feeling heavier or drooping when a character is scared, or a jolt of haptic feedback when something appears suddenly in peripheral vision, can significantly heighten the player's sense of vulnerability and immersion. * **Exploration and Adventure Games:** For games focused on exploring detailed virtual worlds, a strong sense of presence is paramount. When players are encouraged to look around naturally, discover hidden details, and navigate complex environments, accurately simulating the weight, inertia, and subtle movements of the head makes the world feel more solid and real. It enhances the feeling of "being there" and interacting with the environment in a more grounded way. * **Simulation of Physical Tasks and Rehabilitation:** In VR applications designed for training or therapy, such as simulating surgical procedures or performing balance exercises, accurate head simulation is crucial for realism and efficacy. If a user needs to maintain a specific head posture or perform controlled head movements, the simulation needs to provide corresponding sensory feedback to ensure proper form and encourage the desired motor responses. In essence, any VR experience where the player's sense of physical presence, spatial orientation, and interaction with the virtual world is key to the experience will benefit greatly from more sophisticated simulation of the feeling of head.