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
Distribution of receptors Different organs (or organ locations) contain different amounts and types of receptors.
Distribution of receptors
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
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
Spatial frequency Contrast Integration over space.
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
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