Lecture 8 = spatial vision: lines and textures

Question 1

Oriental edges and bars = area of V1 cortex

Answer 1

• Simple cells have receptive fields
• Can map out how receptive fields will respond to particular stimulation
• Structured on/off regions
• 6-8 different orientations and sizes = tuned to light that matches that orientation
• Simple cells divided into R and edge detectors
• Bar detectors can detect edges of a bar of light

Question 2

Orientation selectivity in simple cells

Answer 2

• Receptive fields of V1 cortical cells have ability to detect orientation of stimuli
• Shape of on region aligned with certain orientation of light
• Retinal ganglion cells cant differentiate between horizontal and vertical types of light
• Receptive field properties tuned to size or spatial frequency of light bars

Question 3

Simple cell properties

Answer 3

• Orientation selective
• Some position selective
• Some size selective
• Separate on/off regions
• Length summation

Question 4

Complex cells

Answer 4

• High resting output (no stimulus = more activity)
• Receptive fields defined but unstructured (no on/off receptive field)
• Maintain orientation sensitivity
• Maintain spatial frequency sensitivity
• Many simple cells combined
• When outputs of 3 types of simple cell come together, creates receptive field with overlapping on/off regions
• Orientation selective
• Some size selective
• Not position selective
• Length summation

Question 5

Hyper-complex cells

Answer 5

• Inhibition and end-stopped
• Like complex cells but tuned to length of stimulus
• Can tell how long line will be and its orientation
• Cell turned off by stimulus outside its ‘classical’ receptive field
• 2 inhibitory receptive fields = can provide stop command when bar of light excites middle portion, output will increase and AP will fire –> but as light enters inhibitory field firing rate decreases = how it tells the length

Question 6

What is texture?

Answer 6

• Spatial detail at scale that is finer than what observer currently defines as object scale
• If we are looking at a forest the leaves are texture but if we are looking at a leaf it is an object
• Enables us to see edges of objects defined by changes in their surface

Question 7

Texton theory

Answer 7

• Julesz, 1984
• Textons = defining properties of textures, important in helping differentiate textures
• Textons are:
• -> Elongated blobs (line, rectangles, ellipses) with orientation and length
• -> Line endings
• -> Line crossing
• Broken L’s and T’s each have 6 sextons (2 lines and 4 line endings)
• Crosses have 7 (2 lines, 4 line endings and a line crossing)
• Theory rejected as segmentation based on line crossings and line endings easily disrupted
• Possible for humans to differentiate between 2 textures based upon just orientation

Question 8

Texture segmentation

Answer 8

• Northdurft (1990-1991)
• Texture segmentation based on line crossing and line endings are affected by visual manipulations
• Random variations size, position, luminous disrupt segmentation based on crossings and endings
• Orientation differences and sizes may be only real textons

Question 9

Centre surround cells for orientation

Answer 9

• Vertical stimulus appears and activates on portion of receptive field —> AP generated from cell
• Local contrast are the same = cell cant differentiate from inner and outer portion = less activity from cell
• When there is a local difference between inner and outer portions of cells receptive field = increase in activity of cell = then get inhibitory activity from outer portion of receptive field
• Single opponent cell = orientation sensitive
• Double opponent cell = orientation insensitive

Question 10

Older adults and falls

Answer 10

• Texture processing relies on fine detail = older adults have poor vision
• Stages:
• -> 1st extract texture
• -> 2nd extract texture boundaries
• Texture processing also enables the discrimination of shadows vs surface changes via a similar 2 stage process:
• -> Older adults (and people with dyslexia and autism) have problems at the second stage
• -> Leads to changes in stepping behaviour
• Phenomena in Parkinson disease patients also related to this issue = some experience freezing – whereby become frozen when they approach different things e.g. a doorway
• Clash between feature elements in patient optical flow which clashes with rhythm behaviour = processing of visual stimulus can impact on rhythm of walking = pace interfered with by this visual feature

Question 11

Centre-surrounded for orientation: simultaneous tilt illusion

Answer 11

• Act a bit like the centre-surround cells in the retina = but are sensitive to orientation not luminance
• Tilt-illusion works like simultaneous contrast illusion, but with orientation replacing luminance
• Tilt illusion example of neural code being changed by inhibition over space

Question 12

Does simultaneous contrast work for size?

Answer 12

• Titchener circles/Ebbinghaus illusion
• Inhibition over time = colour aftereffects
• Another illusion where there is interaction between centre and surround based upon the receptive fields of V1 cells

Question 13

Inhibition over time and area V1 - tilt after effect

Answer 13

• Perceived orientation is not determined by that activity of one cell but rather by population of cells sensitive to different orientations: the neural code
• Prolonged viewing of adapting to particular orientation results in inhibition over time
• The response of the active cells is turned down by a process called adaptation or inhibition over time
• This inhibition takes time to take effect and lasts beyond the end of the stimulus
• When a new orientation is seen the remaining inhibition alters the neural code
• The new orientation is not seen properly. Shifted on the opposite direction to the adaptor