Sensory and Motor Mechanisms Overview: Introduction Mechanism of hearing Perception Actions related to perception Introduction Sensation may be defined as the information that we get from a sensory organ. It is, however, different from perception. Sensory information contributes to perception. For example, when we look at a drawing of a cube, what our eyes see is a two dimensional image, but what our brain perceives is a three dimensional object. As one can imagine, motor responses and sensory information are closely related. Voluntary or not, our motor responses depend largely on the sensory input and therefore, it is not surprising that the motor and sensory areas in the brain are situated closely. Source: 6http://www2.fi.edu/exhibits/traveling/identity/psychological.php Here, we present some examples of the mechanisms by which we sense the world around us and react to it. Mechanism of hearing Hearing involves capturing the mechanical energy in sound waves, transmission to sensory organs in the ear and transduction into electrical signals, which can then be analyzed by the brain. These functions are performed by the external 3ear, the middle ear and the inner ear, respectively. The external ear includes the auricle, the ear2canal (also known as external auditory meatus) and tympanum, the eardrum. Source: Principles of Neurobiology by Kandel, Schwartz and Jessell. The middle ear 4consists of three small bones - malleus, incus and stapes, commonly known as hammer, anvil and stirrup due to their shapes. Source: 5http://www.phon.ox.ac.uk/jcoleman/middle_ear.GIF The internal ear or cochlea which has the sound receptors, and as shown below, has 7three fluid filled compartments - scala vestibuli, scala media and scala tympani. Source: Principles of Neurobiology by Kandel, Schwartz and Jessell. The vibrational energy in sound waves causes the tympanum to vibrate. These vibrations are tranferred to the cochlea by the 2three bones in the middle ear. The oval window in the tympanum is pushed back and forth by the stirrup, which acts like a piston. This causes pressure changes in the scala vestibuli, which pass on to the scala tympany through the helicotrema and causes vibration of the basiliar membrane. These pressure changes within the cochlea are releived by the round window. The following is a simplified diagram showing how the movements of the stirrup cause changes in the cochlea. Source: Principles of Neurobiology by Kandel, Schwartz and Jessell. The compressions and rarefactions of the longitudinal sound waves are thus mapped onto the crests and troughs of transverse vibrations of the basiliar membrane. The basiliar membrane has the critical property of being non-uniform. The two ends of the membrane differ in their thickness. The cochlear end is also more taut than the apical end. These properties change continuously along the membrane's length. Due to this property, different frequencies cause maximal vibration at different parts of the membrane. The lowest audible frequencies stimulate the apex of the membrane, while the highest audible frequencies stimulate its base. The mapping of frequencies on the membrane is approximately logarithmic. The tuning curve for cochlear hair is shown below: Source: Principles of Neurobiology by Kandel, Schwartz and Jessell. Thus, different parts of the basiliar membrane respond to different frequencies. The vibrations of the basiliar membrane are converted into electrical signals by a group of specialized cells. The brain then analyzes these signals, resulting in the sense of hearing. Perception Binding Problem: The possibility of building perceptions in sequence of step is extremely low. For example, all the information related to depth, motion, color, shape, orientation etc required for visual perception is processed in distributed fashion. The important questions is that how this information which is handled separately and how it is integrated for visual recognition. Perception of moving objects is related to this problem Grandmother Cell: Sparse or distributive Grandmother cell is a neuron which will correspond to a particular concept or object. Few experiments have shown the evidence for this theory. In some instances a particular neuron will correspond to an object and words associated with that object. For example, activity in a neuron is recorded when subject is shown images of opera house and word "sydney opera house". But cells related to face-recognition are not grandmother/gnostic cells. There is no single cell which corresponds to 1only one face irrespective of transformations of orientation ,size and color. Even the most selective face cells usually also discharge, if more weakly, to a variety of individual faces. Furthermore, face-selective cells often vary in their responsiveness to different aspects of faces.1Different individual faces are much more similar to each other in their overall organization than other kinds of stimuli.1These cells might in fact respond as specialized feature detector neurons that only function in the holistic context of a face construct. Therefore grandmother cell hypothesis is not acceptable in face recognition scenario because then we will need thousands of cell for each face. Evidence of processing of visual information related to color, shape, texture etc. in differents parts of the brain supports distributed hyposthesis.[For face perception follow this link Face Perception] Actions related to perception Changing Direction of gaze: One group of retinal axons travel from the eye superior colliculus, which plays an important role in changing the direction of gaze from one object to another. In superior colliculus visual and motor maps are related to one another. Superior colliculus has layered structure, the superficial layer has a two- dimensional map of the visual world projected on to it, such that each point on the surface is excited by a visual stimulus in a particular part of the world. Motor maps are present in deeper layers corresponding to the visual sensory map residing directly above them. The deeper layers are effectively motor maps in the sense that activity in a particular site in the visual map lies immediately above the site on the motor layers that can generate eye movement precisely to the point in visual space that activated the visual map. The motor layers are responsible for generating nerve impulses in the motor neurons causing the direction of gaze to move very rapidly from one fixed point to another. Shift gaze towards a sound source Brain auditory system guides us to shift our gaze towards sound source by integrating information from the two ears. For frequencies below 1 kHz brain detect minute differences in the arrival time of sound at the two ears. The interaural time difference for a sound arising directly from one side is the maximum. Time difference will approach zero when the source is directly ahead or behind. The maximum interaural time difference that human can detect is 10 microseconds, which is associated with a shift in the position of a sound source by just one degree. Although neurons performs at millisecond precision, brain can do micosecond discrimination using medial superior olive or MSO. Signals originating in the left and right ears converge on individual neurons that only respond when excited simultaneously by signals from the left and right. The system computes difference in the time of arrival of a sound in the two ears because different coincidence-detecting neurons are sensitive to different interaural time delays. Signal are sent in such a way that they reach simultaneously so one of them goes through longer path and thus have a interaural time delay. Actions related to internal sense Our actions are strongly influenced by sensations originating in the body and some of them are generally not conscious. These senses are important because they inform the brain about the position of our limbs, muscle force and length, blood pressure, body temperature, our hunger and thirst, and so on. An example related to this is holding water jug in fixed position although there is significant increase in its weight as water fills in it. Special sensory structures called muscle spindles are incorporated into the fabric of our skeletal muscles and monitor the length of the muscle, which enables muscles to handle increasing loads. As the weight of the glass increases, the biceps will lengthen a little, exciting sensory neurons in the muscle spindles causing them to fire nerve impulses at a higher frequency. This information is transmitted to the spinal cord by sensory axons, which form excitatory synapses with motor neurons that innervate the biceps. Consequently the biceps generate more force, compensating for the increasing load. The sensitivity of the muscle spindles is so high that the desired length of the biceps muscle is restored quickly and the position of the hand is maintained. The spinal cord uses non-reflexive neural circuits for co-ordinating movements of the body and limbs during locomotion. Sensory feedback from muscle and joint receptors analysed the basic pattern, reinforcing it and making adjustments for variations in locomition. References [1] The Brain: A Very Short Introduction by Michael O Shea [2] Principles of Neurobiology by Kandel, Schwartz and Jessell.