BIO 344 - CH 7, 8, 9, 10, 11 & 12 - PDF

# Chapter 9 - Introduction - We are confronted with a vast amount of information from the environment every second. - Only a tiny fraction of this information reaches our brain, and even less is consciously perceived. - Our perception of the world does not reflect the "true" and complete informat...

# Chapter 9 - Introduction - We are confronted with a vast amount of information from the environment every second. - Only a tiny fraction of this information reaches our brain, and even less is consciously perceived. - Our perception of the world does not reflect the "true" and complete information present in the environment. - How do organisms distinguish behaviorally relevant information from irrelevant background noise at the neural level? ## The Umwelt - The *Umwelt* is that part of the environment which is perceived after sensor and central filtering. - Examples: - Honey Bees can see ultraviolet light but not red light. They can see features of many flowers that we are unable to perceive. - Bats use ultrasonic sound for purposes of orientation. ## Releasing Mechanisms & Sign Stimulus - Animals are proposed to have sensory and central filter mechanisms that select those stimuli that are biologically relevant while ignoring others. - This is called *releasing mechanisms*. - A releasing mechanism determines what kind of stimuli an animal responds to by producing an associated behavior. - A *sign stimulus* is a component of the environment that triggers a given behavior. - If the stimulus occurs in the context of social communication, it's called a *releaser*. - In the nervous system, there needs to be a link between the processes of stimulus recognition and of triggering muscle activity responsible for behavior - Tinbergen called this the *Innate Releasing Mechanism*. ## Barn Owl (Tyto alba) - Behavior - Barn owls are able to localize prey solely based on the noise produced by prey. - They hunt predominantly at night. - Owls move through tunnels in grass or snow. - The owl's visual system is limited due to these conditions. - Owls visit a number of observation perches within their territory. - Upon hearing prey, a barn owl turns its head in a rapid flick to directly face the source of sound. - They must locate prey in the horizontal plane (azimuth) and the vertical plane (elevation). ## Barn Owl Anatomy - The owl's eyes are unable to move in the socket. - A flick of the owl's head in response to sound takes 60 msec. - The owl's ears have vertical asymmetry in directional sensitivity. - The left ear is above the midpoint of the owl's eyes and points downward. - The right ear is below the midpoint of the owl's eyes and points downward. - The owl has asymmetry in the arrangement of its facial ruff. - This is very efficient at reflecting high frequency sound. - The facial ruff helps to funnel high frequency sound into the owl's ear canals. - The combined features of the owl's anatomy cause the left ear to be more sensitive to sounds from below, and the right ear to be more sensitive to sounds above. ## Accuracy of Prey Localization - The owl's head is first aligned via sound from a zeroing speaker. - The owl is then stimulated by a second speaker called the target speaker, and its orientation to response is monitored by an electromagnetic angle detector system. - The system consists of two copper wires mounted on the owl's head (search coils). - Two larger coils exist between which the owl is positioned (induction coils). - As the owl turns its head, changes occur in current flow in the search coils. - This allows for the determination of both the azimuth and elevation of head movements. - The barn owl can locate sound within one or two degrees in both azimuth and elevation. - Humans are as good as owls in azimuth, but three times worse in elevation. - The error range for an owl is 9 cm (about the size of a mouse). - The accuracy of prey localization varies with a number of factors. - The frequency range of the sound is important. - Owls are most accurate between 5 and 9 kHz. - Half of the owl's basilar membrane is devoted to the analysis of 5 kHz - 10 kHz (acoustic fovea). - When sound frequencies from 5-10 kHz are detected, they vibrate the owl's basilar membrane. ## Physical Parameters of Sound Involved in Orientation - If one of the owl's ears is completely blocked, it makes large errors in localizing the source of sound. - This suggests that an owl's ability to locate prey depends on the comparison of the signal in both ears. - Plugging the owl's left ear causes its head to orient above and a little to the right of the target. - Plugging the owl's right ear causes its head to orient below and a little to the left of the target. - If one ear is partially blocked, it results in significant errors in determining elevation, but only slight errors in azimuth. - Partial blockage of one ear reduces the intensity of the sound, but does not affect the time of arrival at both ears. - This suggests that *interaural intensity differences* are the principal cues for locating sound elevation. - Additional experiments show that locating sound in azimuth is based on the differences in arrival time of sound at the two ears. ## Barn Owls Analyse Both Interaural Time Difference and Interaural Intensity Differences - Barn owls analyze both interaural time difference and interaural intensity differences to locate sound in azimuth and elevation, respectively. **Pathways in the Brain** | **Pathways in the Brain** | **Neural Algorithm** | |:---|:---| | Optic Tectum <br> External Nucleus | Formation of a joint auditory-visual map | | Time <br> Core <br> Anterior Lateral Lemniscal Nucleus <br> Laminar Nucleus <br> Magnocellular Nucleus | Formation of a map of auditory space <br> Elimination of phase ambiguity <br> Convergence of different frequency channels<br> Convergence of time and intensity pathways <br> Detection and relaying of interaural intensity differences | | Intensity <br> Lateral Shell <br> Posterior Lateral Lemniscal Nucleus <br> Angular Nucleus <br> Auditory Nerve <br> Inner Ear | Detection and relaying of interaural time differences <br> Separation of time, frequency, and intensity data <br> Translation of frequency, time, and intensity cues into nerve signals | ## Brothers & Sisters in Christ - "Brothers & Sisters in Christ" appears on the blackboard. - The text is written in white chalk and states "Mondays 8 PM". ## IC - The image appears to be an enlarged portion of a blackboard. - "IC" is written in white chalk and appears to refer to the Inferior Colliculus in the auditory system. - The IC is part of the midbrain, responsible for processing sound. - "AN" is written in white chalk and may refer to the Angular Nucleus, which processes sound intensity. - "LLDP" is written in white chalk and is likely the abbreviation for the Posterior Lateral Lemniscal Nucleus, which is responsible for integrating information about interaural time delays and intensity differences. -"LN" is written in white chalk and is likely the abbreviation for the Lateral Nucleus, which relay information to the IC. ## Parallel Processing of Time and Intensity Information - The auditory nerve is the link between the ear and the brain. - Axons in the auditory nerve originate from nerve cell bodies in the inner ear. - Different sound frequencies are encoded by different fibers of the auditory nerve. - Encoding of intensity and timing of sound are not segregated yet. - The auditory nerve encodes intensity and timing by varying the rate and timing of action potentials. - Changing sound intensity leads to fibers responding distinctly by changing spike rate. - Fibers also fire a particular phase angle of the spectral component to which they are tuned. - This is known as *phase locking*. ## Cochlear Nucleus - Parallel Processing - Fibers of the auditory nerve innervate cochlear nuclei. - The cochlear nucleus contains two subpopulations of neurons: - The *magnocellular nucleus* is less sensitive to changes in intensity, and demonstrates phase locking. - The *angular nucleus* is sensitive to variations in sound intensity, but does not demonstrate phase locking. ## Laminar Nucleus: Computation of Interaural Time Differences - The laminar nucleus receives input from ipsilateral and contralateral magnocellular nuclei. - Information from both ears converges here for the first time. - The laminar nucleus is crucial for measuring and encoding interaural time difference. - The *Jeffress Model* describes how the laminar nucleus works: - *Delay Lines* are a device that introduce specific delay time in the transmission of a signal. - Delay lines reflect differences in the arrival time at the two ears of an acoustic signal. - *Magnocellular neurons* represent the differences in the arrival time at the two ears. - *Coincidence detectors* receive input from both ears. - The time of transmission of signals varies. - Coincidence detectors fire more strongly if phase-locked impulses reach the detector simultaneously. - The firing rate indicates the direction in azimuth of the sound. - The firing rate is represented by laminar nucleus neurons. ## LLDA & LLDp - LLDA - **Anterior Lateral Lemniscal Dorsal Nucleus** - LLDp - **Posterior Lateral Lemniscal Dorsal Nucleus** - Both synapse at the lateral shells of the Inferior Colliculus (IC). ## Posterior Lateral Lemniscal Nucleus - The posterior lateral lemniscal nucleus receives excitatory input from the contralateral angular nucleus and inhibitory input from the contralateral posterior part of the dorsal lateral lemniscal nucleus. - The posterior lateral lemniscal nucleus computes interaural intensity differences. - The difference between the strength of inhibitory input and that of excitatory input determines the rate at which neurons in the lemniscal nucleus fire. - Neurons in the posterior lateral nucleus are arranged in an orderly fashion within the nucleus. - Neurons are selective for different interaural intensity differences. - Neurons in the left nucleus respond maximally when sound is louder in the left ear. - Neurons in the right nucleus respond maximally when sound is louder in the right ear. ## Lateral Shell: Convergence of Timing and Intensity Information - The *lateral shell* is comprised of the core of the central nucleus of the inferior colliculus and the posterior part of the dorsal lateral lemniscal nucleus. - Neurons in the lateral shell project to the lateral shell. - This is the area where timing and intensity information converge. ## External Nucleus of Inferior Colliculus: Formation of the Auditory Map - The *external nucleus* responds to acoustic stimuli only if sound originates from a restricted area in space. - The brain area that responds to the specific restricted area in space is called the *receptive field*. - Neurons in the left external nucleus have corresponding receptive fields primarily in the right auditory space, and vice versa. - Neighboring space specific neurons have receptive fields representing the neighboring region in space. - This leads to the arrangement of neurons where sound azimuth is arrayed mediolaterally, and sound elevation is mapped dorsoventrally. - This forms the Neural Map of Auditory Space. ## Auditory-Visual Map - The auditory pathways are linked very closely with the retina. - Neurons of the *external nucleus of the inferior colliculus* projects to the *Optic Tectum* (superior colliculus of mammals). - This forms a *joint auditory-visual map*. - Each neuron on the map responds to both auditory and visual stimuli arising from the same point in space. - In the map, the representation of the frontal region of space is greatly expanded, and the location of prey is most accurately identified. - *Field L* tells the meaning of the sound in the auditory cortex. - When an owl wears goggles that shift its vision, it shifts which cell in the *Neural Map* in the owl's brain will fire. - **ICX** is the external nucleus: where all calculations are done projecting to the optic tectum. - The *Optic Tectum* is the area where the visual information converges. - The optic tectum is located in the superior colliculus. - The optic tectum synapses to the *Ov* (Nucleus Ovidalis) and then Field L, which deciphers the meaning of the sound in the auditory cortex. ## Blackboard Diagram - The image is an enlarged portion of a blackboard. - The image shows the pathway of a sound signal through the auditory system, from the cochlear nucleus to the Optic Tectum (OT). - The cochlear nucleus is labeled "Cochlear Nucleus" and is located at the bottom of the diagram. - The inferior colliculus is labelled "IC". - The lateral lemniscus (LN) is labelled "LN" and the posterior lateral lemniscus (LLDP) is labelled "LLDP". - The lateral shell is labelled "Lateral Shell (IC)" and the posterior lateral lemniscus is labelled "Pos Lateral Lemnisal Nucleus" - The angular nucleus is labelled "AN" and the magnocellular nucleus is labelled "MN". - The optic tectum is labelled "OT" and the nucleus ovoidalis (Ov) is labelled 'Ov'. - The field L is labelled “Field L”. - Arrows indicate the direction of information flow from the auditory nerve to the Optic Tectum (OT). - "Meaning" appears at the top of the diagram, indicating that the Optic Tectum is responsible for interpreting the meaning of the sound processed by the auditory system. - "Retina" appears at the bottom of the diagram, indicating that the auditory system is linked to the visual system. - The diagram also has the following labels: - "8 -> Auditory Nerve" indicating that the auditory nerve is cranial nerve number eight. - "S-10kHz" indicating the frequency of sound that is being processed in the auditory system. ## Adaptations of the Auditory Sensory System - In most mammals, the frequency of sound is encoded by the displacement of the basilar and tectorial membranes at specific places. - The representation of frequencies on the basilar membrane around 83 kHz is greatly expanded for both length and thickness. - An *acoustic fovea* is a specialized region of the inner ear with a disproportionate number of receptor cells, sharply tuned to a very narrow and behaviorally important frequency range. ## Auditory Cortex of Mustached Bats - The FM-FM area processes information related to echo delays. - Neurons in the FM-FM area respond poorly when presented with a pulse, echo, CF signal, or FM sweep alone. - Neurons in the FM-FM area respond vigorously when a sound pulse is followed by an echo at a particular delay. - Neurons in the FM-FM area are arranged in a topographic fashion. - Delay time increases along one axis. - The range varies from 0.4 msec to 18 msec, corresponding to 7 and 310 cm target distance. - The *computational map* plays a key role in information processing by the CNS. - The values for a computed parameter (pulse echo delay) vary systematically across at least one dimension of neural structure. ## Adaptations of the Auditory Cortex - The *Auditory Cortex* has overrepresentation by acoustic fovea accompanied by high density of innervation of the cochlea by first-order neurons. - It also contains CF area and FM area. - Each part of the echo is represented by two distinct areas in the auditory cortex. - Different types of information about the echo are processed separately. - The *Doppler Shift Constant Frequency Area (DSCF)* specializes in detecting rapid Doppler modulations (wing flutter) to identify the target. - The *FM-FM area* responds to echo delay or target range (distance). - The *CF-CF area* responds to Doppler magnitude (target velocity). - The *azimuthal* indicates the vertical location of the target.