Lecture 3

Question 1

Object recognition involves three main stages.

Answer 1

Perception of features - what is really there?
Perception of patterns - what parts belong to the object? which to the background?
Recognition - what is the object?

Question 2

Stage 1 - Perception of features

Answer 2

our eyes take light in and it lands on the retina
light can be lighter or darker depending on the object
identifying the difference in light is known as the luminance edge
edges are the first available feature of vision, they signify a change

Question 3

what detects edges?

Answer 3

ganglion cells

Question 4

ganglion cells detect edges

Answer 4

all edges are treated as equals by the ganglion cells, regardless of whether they belong to objects or shadows

Question 5

striate cortex

Answer 5

primary visual cortex
we have the information but not the picture yet

Question 6

feature detection in the brain

Answer 6

the primary visual cortex has cells that specials in the detection of:
motion
colour
orientation
size

Question 7

what evidence do we have for the existence of specialised cells?

Answer 7

The brains of other species contain cells with the properties of feature detectors.
After staring at certain patterns, we see a aftereffects that implies fatigue of
feature-detector cells in the human brain

Question 8

Hubel and Wiesel

Answer 8

inserted thin electrodes into cells of the occipital cortex of cats and monkeys and then recorded the activity of those cells when various light patterns struck the animals’ retinas.
At first, they used mere points of light, which produced little response. Later they tried lines.

Question 9

what did Hubel and Wiesel find?

Answer 9

Individual cells respond best in the presence of a particular stimulus
Some cells activate only when a vertical bar of light is presented.
Others become active only for a horizontal bar.
Later they found cells that respond to other features, such as movement in a particular direction.
In other words, such cells appear to be feature detectors

Question 10

simple cells

Answer 10

have elongated receptive fields
this makes them maximally sensitive to a particular line or edge of a particular orientation at a particular location of the retina

Question 11

complex cells

Answer 11

respond strongly to lines of a particular orientation moving in a particular direction

Question 12

hypercomplex cells

Answer 12

respond best to lines of a particular length, and moving in a particular direction
also respond to moving corners or angles
some will fire when a line ends in their receptive field, so they are good size detectors as well

Question 13

motion aftereffect

Answer 13

suggest that we have cells that specialise in detecting
the motion direction of lines.
our brain begins to make stuff up as they are too tired to work properly

Question 14

perception of patterns

Answer 14

Our ability to perceive something in
more than one way is the basis of
Gestalt Psychology, a field that focuses
on our ability to perceive overall
patterns.
Perception cannot be
broken down into its component parts.

Question 15

bottom up and top down

Answer 15

the detection of patterns - how we perceive things e.g letter
top down information - the expectation surpass the information

Question 16

proximity

Answer 16

the tendency to perceive objects that are close together as belonging to a group. The objects in
part a form two groups because of their proximity.

Question 17

similarity

Answer 17

The tendency to perceive objects that
resemble each other as a group

Question 18

principle of good continuation

Answer 18

when lines are interrupted, we perceive continuation, a filling in of the gaps

Question 19

principle of closure

Answer 19

when a familiar figure is interrupted by another object, we perceive a closure of the hidden part of the figure

Question 20

principle of good figure

Answer 20

when possible, we tend to perceive a good figure - a simple, familiar, symmetrical figure

Question 21

where does grouping happen in the brain?

Answer 21

The features extracted from the retinal image are grouped in a way that will allow us to know which lines and colours and motions belong to which object out there.
Grouping is mainly done in structures further down the ventral or ‘what’ processing stream.

One important area along
the ventral visual stream is
the Lateral Occipital Cortex
(or LOC).

Question 22

Visual apperceptive agnosia

Answer 22

Damage to the LOC following stroke or
carbon monoxide poisoning, leads to
visual apperceptive agnosia –inability to
perceive any object through vision.
Apperceptive agnosia is not blindness.
Patients can only detect simple features,
e.g., lines, the presence of closure, but
they cannot perceive (see) even the
simplest forms.
usually tested using copy drawing and perceptual matching task

Question 23

bottom up processes are necessary in perception

Answer 23

sensory information is
analysed in our sensory
systems starting at the
sense organs, and in
corresponding brain areas.

Question 24

top down processes are also necessary

Answer 24

At the same time our
knowledge, beliefs,
expectations, affect
processing from the top down
–those lead to assumptions
about what we are seeing.