wk 4 SP - visual cortex Flashcards
Ganglion cell fibres leave
retina along _________ _________
optic nerve
Beyond optic
chiasm the optic
nerve becomes
the ______ _______
optic tract
Optic Tract
info now separated by _____ _____rather than by ______
visual field
eye
LGN =
Lateral Geniculate Nucleus
the optic tract feeds into the
LGN
LGN = bilateral
structure meaning
(one in left
hemisphere and one
in right)
Each LGN receives
input from left and
right eyes – but keeps
these inputs
separate
LGN Receptive Fields
LGN cells have the same
receptive field organisation
as retinal ganglion cells:
centre-surround
antagonism.
Ideal for detecting spots of
light & edges
* But NOT able to detect
orientation of lines/ edges
Objects close
together in the
visual scene are
analysed by
(neighbouring/far) parts
of V1.
choose one
neighbouring
Amount of cortex
devoted to
representing each part
of the retinal field is
distorted, e.g:
Fovea represented by (small/large) area of cortex
choose one
large
Fovea accounts of _____% of the
retina but is represented by___ - __ % of V1
0.01%
8-
10%
do V1 cells have a level of baseline activity when no stimulus is presented
yes
V1 cells prefer lines of a
particular orientation
3 different types of cell in V1 each with distinct receptive field organisation
- simple cells
-complex cells
-Hypercomplex cells
Simple Cell Receptive Fields
Simple cells respond to
oriented lines and
edges
The receptive field has
excitatory and inhibitory
regions, but they are
elongated
e.g. if the excitatory region is vertical, a vertical line would cover the excitatory and not the inhibitory, causing a big excitatory response
whereas a slanted line would cover inhibitory regions, causing a weaker response
Search image up on google to get better idea
Orientation tuning
similar orientations
Orientation tuned neurons
respond best to their
preferred orientation but
also respond to other
similar orientations
different types of simple cells (v1)
Some simple cells have ON-centre RFs and some have
Off-centre RFs, but all have a preferred orientation
Complex Cell Receptive Fields
Respond to oriented lines but no discrete ON
and OFF regions
respond to moving oriented lines and edges
- respond best to a particular direction of
movement
Hypercomplex Cell Receptive Fields
Also called End-stopped cells
* Respond to lines of:
– particular orientation AND
– moving in a particular
direction AND
– particular length
Areas seem to be
specialised e.g.
– v3 =
– v4 =
– v5 =
form
colour
motion
Processing streams: What vs. Where
‘What’ stream
Travels ventrally to inferotemporal cortex
* Important for recognising and
discriminating objects
Processing streams: What vs. Where
‘Where’ stream
Travels dorsally to posterior parietal cortex
* Important for determining where an object is and how to
act upon it
* Sometimes referred to as the ‘How’ stream
Optic ataxia
Damage to dorsal pathway (‘where/ how’ stream):
* Cannot reach to grasp objects, but can recognise
and describe them
* Opposite deficits to those shown patients with
visual form agnosia (e.g. patient DF)
Visual form agnosia
Damage to ventral pathway (‘what’ stream):
* Cannot identify objects despite knowing their features
