Hearing and Vision: Lecture Notes PDF

The Ear as a Frequency Analyser Physical properties of objects, such as size, mass, stiffness, are reflected in the frequency spectra they emit when they make sounds. An important job of the ear is to perform a time- frequency analysis of incoming sounds. This results in: A place...

The Ear as a Frequency Analyser Physical properties of objects, such as size, mass, stiffness, are reflected in the frequency spectra they emit when they make sounds. An important job of the ear is to perform a time- frequency analysis of incoming sounds. This results in: A place code for frequency (tonotopy) A rate code for intensity A time code for temporal structure (including “fine structure”)  Anatomy of the Ear Outer ear: entry of sound waves – Pinna – Ear canal Middle ear: amplification of sound waves – Tympanic membrane – Ossicles: malleus, incus, stapes – Oval window – Round window Inner ear: Cochlear  Mechanoelectrical transduction mediated by hair cells When the hair bundle is deflected toward the tallest stereocilium (Kinocilium), cation-selective potassium channels open near the tips of the stereocilia, allowing K + (high K+ but low Na+ in Scala media) to flow into the hair cell down their electrochemical gradient. The resulting depolarization of the hair cell opens voltage-gated Ca2+ channels in the cell soma, allowing calcium entry and release of neurotransmitter onto the nerve endings of the auditory nerve.  The structure of the hair bundle in cochlear hair cells Scanning electron micrograph of a cochlear outer hair cell bundle. Tip links that connect adjacent stereocilia are believed to be mechanical linkages that open and close transduction channels.  Basilar membrane mechanics: a trade-off between stiffness and flexibility The basilar membrane is stiff at the base and floppy at the apex. Vibrations travelling from the stapes to the round window must either take a short route through stiff membrane or a longer route through floppy membrane.  Signal coding for Sound Coding for the qualities of sound – Intensity (rate) coding = loudness (amplitude, decibel) Coded based on the degree of deflection and opening of ion channels in stereocilia – Frequency (place) coding = pitch (Hz) Coded based on the hair cell location on the basilar membrane where the deflection occurs – Time coding = temporal discharge pattern of signals Phase locking enables a timing code in the local population  Auditory Pathways Hair cells (in the Cochlea) synapse on afferent axons (spiral ganglia) of cochlear nerve VIII Cochlear nerve enters brainstem (cochlear nucleus - superior olivary nucleus - inferior colliculus) – thalamus (medial geniculate nucleus) – auditory cortex Auditory cortex (temporal cortex) has a frequency (tonotopic) map which is inherited from the cochlea  Anatomy of the retina Reflection – We perceive light waves reflected off objects Cells of the retina – Rods and cones are photoreceptor cells – Rods and cones communicate with bipolar cells – Bipolar cells communicate with ganglion cells – Axons of ganglion cells form the optic nerve – Horizontal and amacrine cells provide lateral inhibition Structure of the retina cells Structural differences between rods and cones. Although generally similar in structure, rods and cones differ in size and shape, as well as in the arrangement of the membranous disks in their outer segments. Rods: Dim light, sensitive to low light Cones: Day light, color and high acuity Phototransduction in rod photoreceptors (A) Rhodopsin resides in the disk membrane of the photoreceptor outer segment. The seven transmembrane domains of the opsin molecule enclose the light-sensitive retinal molecule. (B) Absorption of a photon of light by retinal leads to a change in configuration from the 11-cis to the all-trans isomer. (C) The second messenger cascade of phototransduction. The change in the retinal isomer activates transducin, which in turn activates a phosphodiesterase (PDE). The PDE then hydrolyzes cGMP, reducing its concentration in the outer segment and leading to the closure of channels in the outer segment membrane (light-induced hyperpolarization).  Dogs are dichromatic The trick to seeing color is not just having cones, but having several different types of cones, each tuned to different wavelengths of light. Human beings have three different kinds of cones and the combined activity of these gives humans their full range of color vision. Dogs have fewer cones (yellow and blue) than humans which suggests that their color vision won't be as rich or intense as ours. What color of dog toy should you buy for your dog?  Neural Pathways for Vision Ganglionic cells – Optic nerve (cranial nerve II) – Optic chiasm – Optic tract Lateral geniculate body of thalamus synapses Optic radiations Visual cortex synapses Right visual field to left cortex, and vice versa  Beyond the primary visual cortex (VI) Parallel pathways transfer different types of visual information Dorsal stream – Analysis of visual motion and the visual control of action (Movement & Depth) Ventral stream – Perception of the visual world and the recognition of objects (Color & shape)