PLASTICITY IN SENSORY CORTEX
- Plasticity of primary cortical areas suggests that there are procedure-like "sharping " or
"tuning" mechanisms that operate throughout the life time of the animal to change the
receptivity of the individual processing units within the basic processors of the brain.
- SELECTIVE EARLY EXPERIENCE AND CRITICAL PERIODS
- restricting early visual experience in kitten, by arranging for input to be provided
by one eye only
- Cells in primary visual cortex show a preference in the relative balance
given to input from the two eyes, see Fig. 3-8
- The remaining eye takes over the cortical target area
- restricting early visual experience in kitten to contours of certain orientation, see
- Individual visual cortex neurons change their orientation selectivity in
conformance with their experience, see Fig. 3-10
- PLASTICITY IN ADULT SENSORY CORTEX
- A rapidly growing literature reveals a very large degree of short- and long-term
modification of receptive field and reorganization of representation maps
- Gilbert et al: Receptive field dynamics in adult primary visual cortex
- Background information
- the concept of receptive field
- sensory cortex is topographically mapped, with retinal coordinates
(or body surface) mapped systematically on the visual (or
- In visual cortex, retinal lesions lead to reorganization of cortical
topography in the long term recoveries, see Fig. 3-11
- Within minutes of the lesion procedure, cortical cells with receptive fields
located near the boundary of the lesion expand in size, see Fig. 3-12.
- After a few months even the cortical areas that were initially silenced by
the lesion recover visual activity, representing retinotopic loci surrounding
the lesion, see Fig. 3-13.
- Cortical changes under normal sensory and behavioural experience
- "artificial scotoma": the area surrounding the receptive field was
stimulated visually, but the receptive field itself was masked with a
visual occluder, see Fig. 3-14.
- After a 10-minute period of conditioning the cell with this
stimulus, its receptive field expanded severalfold in length, see
Fig. 3-15, 3-16, 3-17.
- The results suggested an ongoing process of modulation of
receptive field size, adapting in different ways to different scenes.
- the short-term plasticity must involve in some way a change in the
synaptic weight of existing connections, altering the patterns of activation
of intrinsic cirtuits.
- It seems likely that mutability of receptive fields and cortical architectures
is associated with normal sensory experience, not just peripheral lesions,
and that the changes can take place on a brief time scale of minutes.
- Merzenich et al: rearrangements of the somatotopic representation (Fig. 3-18, 3-19) in primary somatosensory cortex of monkeys following a variety of traumatic
and more natural interventions in somatosensory input.
- Following digit removal, the cortical representation of neighbouring digits
invades the cortical zone whose afferents have been removed, see Fig.
- Surgical joining of the digits results in the establishment of a continuous
somatic representation of formerly discontinuous zones for each digit.
- Tactile discrimination training (for several weeks) results in altered
sensory receptive fields in the somatosensory cortex (larger cortical
represenations of the stimulated digits, and larger receptive fields in the
- Synchronization of cortical physiology acquired during training is well
correlated with the behavioural performance in a discrimination task.
VISUAL MEMORY -- INFEROTEMPORAL (IT) CORTEX
- Visual pathway,see Fig. 3-21
- Kluver and Bucy (1937): psychic blindness
- Removal of temporal lobe of the monkey led to the lost of ability to recognize
familiar objects and their biological significance
- Human defect: visual agnosia
- Patients with visual agnosia can see all parts of the visual field, but the objects
they see mean nothing to them
- Poor picture memory, difficulty learning new faces
- Face cells in inferotemporal cortex
- Gross et al (1972), Response to hands and faces, see Fig. 3-22, 3-23