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Sensory systems basics

Sensing the external world

Sensory transduction Transforming external physical forces/energy into electrical impulses that are mediated by neural spikes. Neural “encoding”

Encoding stimulus amplitude Responses of a photoreceptor to light at different intensities.

Information is encoded by both amplitude and length of the response.

Encoding stimulus location The location of the stimulus in space is encoded by the identity of responding receptors.

Encoding stimulus location

Encoding stimulus location

Distribution of receptors Different organs (or organ locations) contain different amounts and types of receptors.

Distribution of receptors

Distribution of receptors

Sensory pathways

Sensory pathways

Central nervous system

Vision

Topographical organization Photoreceptors responding to the left visual field innervate the right LGN.

Topographical organization Both the thalamus and early visual cortices contain retinotopic maps of visual space.

Occular dominance Information from the left and right eyes remains segregated in the LGN.

Occular dominance Also in primary visual cortex.

Cortical magnification

Cortical magnification

Acuity

Columnar organization

Selectivity to stimulus attributes Spatial receptive fields Contrast Luminance Spatial frequency Orientations Colors Movement direction and/or speed Textures Shapes

Receptive field Many visual neurons have excitatory and inhibitory parts to their receptive field. Examples of retinal and LGN cells.

Retino-topic mapping

Luminance & Contrast

Orientation selectivity Orientation selectivity in primary visual cortex.

Orientation selectivity

Pinwheels (only in primates)

Hierarchy and integration LGN

V1 Neurons

Invariance and Gain

Contrast invariant orientation tuning. Response gain

Contrast

Spatial frequency

Integration over space.

Spatial frequency

Movement direction Integration over time.

Overlapping representations of orientations and directions

Hierarchy and integration

Low, mid, and high level vision

Functional specialization

Face selectivity

Invariance (abstractness)

Is this vision or abstract memory?

Audition

Choclear output Selectivity to specific frequencies. Louder stimuli generate less selective responses.

Sound localization

Interaural time differences (ITD)

Sound localization

Brainstem areas: Olivary nuclei Colliculus

Sound localization

Auditory brainstem response

Tonotopy

But no spatial encoding…

Language system

Lateralized!

Language structure Phonetics: ‘ba’, ‘da’, ‘pa’ Words, Grammar, Intonation How is all this encoded? Language hierarchy?

Invariance across hearing and reading?

Specific white matter tracts

Aphasias

Somatosensation

Parallel pathways

Parallel pathways

Each pathway conveys a different “part” of the information. Redundancy…

Topographic organization

Homonculus

Secondary Somatosensory areas

How does all this develop?

Genetics Experience Flexibility/Plasticity