Physics of Sound Signals and Ear Mechanics

Sound Signals (Impulse Responses)

• Sounds are pressure waves in the air that carry information about emitting objects.
• The signal emitted by an object when hit is called the impulse response.
• Sine waves are fundamental components of complex sounds.

The Ear as a Frequency Analyzer

• The ear analyzes the frequencies in sound to determine the object's physical properties (size, mass, stiffness).
• The ear creates:
  • Place code for frequency (tonotopy): different frequencies activate different areas of the basilar membrane.
  • Rate code for intensity: the intensity of the sound is coded by the firing rate of auditory neurons.
  • Time code for temporal structure: the timing of neural firing allows for the perception of fine details in sound.

Basilar Membrane Mechanics

• The basilar membrane is stiffer at the base and more flexible at the apex.
• Traveling waves are created when sound vibrates the basilar membrane.
• Higher frequencies vibrate the membrane near the base, while lower frequencies vibrate the membrane near the apex.

Signal Coding for Sound

• Intensity (rate) coding: loudness is coded by the degree of deflection and opening of ion channels in stereocilia.
• Frequency (place) coding: pitch is coded by the location of hair cell activation on the basilar membrane.
• Time coding: temporal discharge patterns of signals allow for the perception of fine detail in sound.

Auditory Pathways

• Hair cells in the cochlea synapse on the afferent axons of the cochlear nerve VIII.
• The cochlear nerve interacts with the brainstem (cochlear nucleus, superior olivary nucleus, inferior colliculus).
• The information then travels to the thalamus (medial geniculate nucleus) and finally to the auditory cortex.
• The auditory cortex has a tonotopic map, meaning different frequencies are processed in different areas.

Cochlear Implant: Engineering for Compensation

• Cochlear implants electrically stimulate the auditory nerve to restore hearing in people with severe hearing loss.
• They consist of an external microphone, processor, and transmitter, as well as an internal processor and electrodes implanted in the cochlea.

The Human Eye

• Only 1/6 of the eyeball is exposed.
• The cornea is the only tissue without blood vessels.
• Eye muscles are the most active muscles in the body.
• The iris has 256 unique characteristics, making retina scans more secure than fingerprint scans.

Anatomy of the Retina

• The retina is a circular disc, approximately 10 square centimeters in area.
• It is a thin sheet of neurons, about 200 µm thick.
• Cones are concentrated at the fovea, responsible for high-acuity color vision.

Cells of the Retina

• Rods and cones are photoreceptor cells that detect light.
• Bipolar cells receive input from rods and cones and transmit information to ganglion cells.
• Ganglion cells send signals to the brain via the optic nerve.
• Horizontal and amacrine cells provide lateral inhibition, which enhances contrast and sharpens edges.

The Blind Spot

• The optic disc is the point where the optic nerve exits the eye, lacking photoreceptor cells.
• Binocular vision and "filling-in" processes compensate for the blind spot by using information from both eyes.

Phototransduction in Rod Photoreceptors

• Light absorption by rhodopsin in the outer segment of rods initiates a cascade of events.
• This leads to the closure of sodium channels and hyperpolarization of the photoreceptor.

Opsin Proteins for Trichromatic Perception

• Three types of cones with different opsin proteins (L-cones, M-cones, and S-cones), each sensitive to different wavelengths of light, enable color vision.

Why are some people color blind?

• Color blindness is caused by mutations in opsin genes.
• Red-green color blindness is the most common form, followed by blue-yellow color blindness.
• Red-green color blindness is more common in males because the genes for red and green opsin are on the X chromosome, of which males only have one.

Ganglion Cells

• Ganglion cells emphasize moving objects and temporal changes in visual input.
• Microsaccades, small involuntary eye movements, prevent the world from disappearing when we fix our gaze.

Troxler Effect

• When there are no microsaccades, a stabilized image on the retina fades from view within seconds.

Retinotopic Map in the Visual Striate Cortex

• The cortex contains a map of the visual field where different areas respond to different visual locations on the retina.

Depth Perception

• Binocular vision (using both eyes) contributes to depth perception.
• Memories of past experiences and the brain's ability to analyze depth cues in the visual scene also play a role.

Neural Pathways for Vision

• Ganglion cell axons form the optic nerve.
• The optic nerve crosses at the optic chiasm, with the right visual field projecting to the left hemisphere and vice versa.
• Signals travel to the lateral geniculate body of the thalamus and then to the visual cortex via optic radiations.

Beyond the Primary Visual Cortex (VI)

• Dorsal stream: analyzes visual motion, spatial location, and the visual control of action.
• Ventral stream: processes object recognition, color, and shape.

Explore the fascinating concepts of sound signals and how the ear functions as a frequency analyzer. This quiz delves into impulse responses, the mechanics of the basilar membrane, and the coding strategies utilized by the auditory system. Test your knowledge on these fundamental aspects of sound and hearing.