WEEK 6 - Sensory Organization and Processing

Sensory Systems Overview

• Principles of Sensory System Organization

  • Three types of sensory cortex:

    • Primary sensory cortex (one sense)

    • Secondary sensory cortex

    • Association sensory cortex (multiple senses)

  • Hierarchical organization: Sensory signals flow through brain structures moving from high resolution/low interpretation to high interpretation. Sensation is less complex than perception.

  • Functional segregation: Different parts of the brain specialize in different kinds of analysis of sensory information. The hierarchy in cortex goes from primary to secondary to association.

  • Parallel processing: Information flows between structures simultaneously along multiple pathways.

Auditory System

• Physical and Perceptual Dimensions of Sound

  • Amplitude: Loudness

  • Frequency: Pitch (related to fundamental frequencies)

  • Complexity: Timbre

  • Characterized by Fourier Analysis.

  • Dimensions refer to effects of stimuli on air, creating air movement (waves).

• Anatomy of the Ear

  • Outer ear

  • Tympanic membrane (ear drum)

  • Middle ear

    • Ossicles (Hammer, anvil, stirrup)

  • Inner ear

    • Oval window

    • Organ of Corti

    • Hair cells of basilar membrane

    • Tectorial membrane

• Pathway from Inner Ear to Brain

  • Hair cells synapse on neurons.

  • Axons enter hindbrain.

  • Synapse in ipsilateral cochlear nucleus.

  • Travel to superior olives.

  • Travel to the inferior colliculus.

  • To the medial geniculate nuclei of the thalamus.

  • To the primary auditory cortex.

• Sound Localization

  • Superior olives

    • Medial superior olives: Neurons respond to differences in time arrival of sounds.

    • Lateral superior olives: Neurons sensitive to differences in loudness in both ears.

  • Inferior colliculus

    • Receive inputs from olives.

    • Tonotopically organized.

      • Tonotopic: Neurons are physically located in the brain structure according to the frequencies to which they respond.

• Auditory Cortex

  • Input from medial geniculate nucleus (thalamus) to primary auditory cortex.

    • Primary auditory cortex: tonotopically organized, processes tone and pitch, then projects to secondary auditory cortex.

  • Secondary auditory cortex: not tonotopically organized; processes complex tones and encodes sound localization.

• Auditory Pathways: Signals projected as two streams into other parts of cortex

  1. Anterior auditory pathway: Sound identification.

  2. Posterior auditory pathway: Sound localization.

Visual System

• Environmental Feature Conversion

  • Light enters the eye and reaches the retina.

  • Retina translates light into neural signals.

• Pupil and Lens

  • Pupil: Size regulated by iris (compromise between sensitivity and acuity).

  • Lens: Focuses light on the retina (accommodation).

    • Near focus: Lens is cylindrical.

    • Far focus: Lens is flattened.

• Retina Structure

  • Five Layers: Receptor Layer, Horizontal Layer, Bipolar Layer, Amacrine Cell Layer, Retinal Ganglion Cell Layer

  • Fovea: High-acuity vision (axons of retinal ganglion cells are thinnest over the fovea).

  • Optic disk: Blind Spot

• Duplexity Theory

  • Rods and Cones Mediate Different Types of Vision

    • Photopic (cone-mediated): High acuity in good lighting.

    • Scotopic (rod mediated): Low acuity in dim lighting, lacks detail & color.

• Photoreceptors: Rods and Cones

  • When light hits the rods, it causes ‘bleaching’ of the Rhodopsin. This induces a hyperpolarization of the rods.

  • Rods contain rhodopsin, cones have sensitivity to colors (blue, green, red).

  • \sim 120 million rods, 6 million cones

• Eye to Retina to Geniculostriate Pathway

  • Retina -> Geniculate (thalamus) -> Striate Cortex (occipital lobe, primary visual cortex).

  • Pathway: Nasal hemiretinas decussate at optic chiasm, temporal hemiretinas stay ipsilateral.

• Retinotopic Layout

  • The surface of the visual cortex is a map of the retina.

• M and P Channels

  • Magnocellular Layers: Bottom two layers of lateral geniculate nucleus, large body neurons, responsive to rods and movement.

  • Parvocellular Layers: Top four layers of lateral geniculate nucleus, small-body neurons, responsive to color, fine detail, slow/stationary objects.

• Lateral Inhibition for Contrast Enhancement

  • Edges are important for perception; the visual system enhances contrast.

  • Mach Bands Illusion: Light areas near border appear dark.

  • Mechanism:

    • Activation of receptors at one edge inhibit activation of adjacent receptors.

    • Receptors near edge receive more lateral inhibition.

    • Receptors near the edge of the bright side receive less inhibition.

• Receptive Fields of Visual Neurons

  • A neuron’s receptive field is a characteristic of a visual field that a neuron is responsive to.

  • Hubel and Wiesel methodology for single neuron recordings

    • Visual stimuli presented on a screen

    • Subject is curarized (treat with curare), so eyes don’t move

    • Extracellular electrode placed to record only one neuron

    • Then various stimuli are presented

    • Scientists watched for what type of stimuli activated the neuron (that is, increased it’s firing)

    • In this way, an individual neuron’s receptive field is mapped

    • H&W systematically recorded from neurons along the visual pathway to discover what stimuli they were receptive to

• Receptive Fields in Retina-Geniculate-Striate System

  • Similar receptive fields are found in retinal ganglion cells, lateral geniculate nucleus neurons, and lower layer IV of striate cortex.

  • Two patterns of responding:

    • On firing

    • Inhibition followed by off firing

  • Neurons respond best to fully illuminated “on” area, respond poorly to diffuse light, and respond to brightness contrast.

• Primary Visual Cortex Organization

  • Neurons respond to line orientation, line movement, periodic patterns, spatial gradients, texture.

  • Specializes in feature detection in small portions of visual field; it creates piecemeal elements of information.

• It retails localization information in relation to the visual field by virtue of spatial retinotopic mapping

• Receptive fields of these neurons change based on context

Cortical Processing

• Visual Areas of Cortex

  1. Primary visual cortex: Input from the lateral geniculate nuclei.

  2. Secondary visual cortex: Includes prestriate cortex and inferotemporal cortex, input from primary visual cortex.

  3. Posterior parietal cortex: Input from the secondary visual cortices for localizing objects in space.

  4. Inferotemporal cortex: Information on object features for object identification.

• Dorsal and Ventral Streams

  • Dorsal stream: Information flows from primary visual cortex, travels through dorsal prestriate secondary visual cortex, ends in association cortex of posterior parietal region (behavioral control path).

  • Ventral stream: Information flows from primary visual cortex, travels through the ventral prestriate secondary visual cortex, ends in association cortex of posterior parietal region (conscious perception pathway).

• Prosopagnosia

  • Inability to recognize faces.

  • May involve neurons of inferotemporal cortex.

  • Damage to fusiform face area.