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Motion Perception 2

PSY305 Lecture 13 JV Stone

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StructureThe role of area MT in motion perception

1. Acquired lesions (Zihl et al. 1983, Schenk & Zihl, 1997)

2. Human PET (Zeki et al. 1991)

3. Single cell recording (Britten et al. 1992)

4. Microstimulation (Salzman et al. 1992)

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Cells in area V1 (striate cortex) respond to localmotion

• Direction sensitive cells havebeen identified in layer 4B ofthe striate cortex, and sufferfrom the aperture problem,and respond only to localmotion.

• However, cells in area V5(MT) are able to solve theaperture problem. They canrespond to global motionsignals rather than localmotion signals.

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1983: Well documented case of akinetopsia(inability to see motion).

For a historical perspective of this see Zeki (1993) Chapter 10. Formore detail, read Zihl et al. (1983) Brain, 106, 313 -340.

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Is there a human motion area?

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Evidence using PET in humans

• Compared brain activitywhen looking at staticmotion vs black and whitestatic.

• Difference in activity largestin MT (V5).

Zeki et al. 1991, J of Neuroscience, 11, 641 - 649

Static vs motion

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Motion aftereffect

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Motion aftereffect

• After staring at the downward motion of awaterfall, static objects appear to move upwards.

• This illusion suggests that the human visualsystem is equipped with pairs of directionallyselective motion detectors which act inopposition to each other …

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Motion aftereffect

Postulated neuronal firing patterns following adaptation to leftward motion.

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Neural adaptation to motion

Adaptation to motion in retinalganglion cell of rabbit’s retina.Falling response to constantstimulation during motion mayunderpin perceived reduction inmotion over time.Sub-baseline firing rate aftermotion ends may underpin aftereffect, eg if perceived motiondepends on comparing output ofcells with opposite preferreddirections.Barlow and Hill (1963).

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Comparing monkey to responses of neuronsin area MT (V5)

Britten, Shadlen, Newsome, & Movshon (1992).

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Each cell has a preferreddirection. Having found thepreferred direction of acell, then present it’sreceptive field with stimulionly in preferred or nulldirection but at varyinglevels of correlation …

Preferreddirection

Preferred direction

Nulldirection

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Cell response to inreasing correlation inpreferred direction

Single cell recording method Schematic illustration of results

0% correlation

50% correlation

100% correlation

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What the monkey saw (and when)

2 seconds

Disc corresponds to receptive field of MT neuron.On each trial:

1 Monkey looks at fixation point, then …2 Stimulus appears for 2 seconds, then …3 Target lights (LEDs) switched on, then …4 Monkey fixates LED for preferred or null direction.

Null LED

Preferred LED +

Aperture

Receptive field

Fixation point

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Correlated dot motion

On each trial, a proportion ofdots move coherently together.The more dots that movetogether, the easier it is toidentify the direction of motion.

0% correlation 50% correlation 100% correlation

Receptive fieldof neuron

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Psychometric function using 2AFC• Direction of motion is easy to detect (100%) correct when the

dots are 100% correlated (coherent).• When dot motions are uncorrelated, the correct LED is

chosen on 50% of trials.• As more dots are correlated, the task becomes easier, until

almost perfect performance is reached …

% c

orre

ct

correlation

95

75

50

200 10

Null LED

Preferred LED +

Aperture

Receptive field

Fixation point

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Now to compare …

• Psychometric function: the change inpreferred/null choices made by monkey as dotcorrelation increased, using eye movements to indicateperceived direction.

• Neurometric function: the change inpreferred/null choices that would be made by astatistically optimal decision mechanism based onthe firing rate of a single cell.

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Psychophysical Result

0% correlation 50% correlation 100% correlation

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Corre

latio

n

Open bars represent cell responsesto stimuli in the preferred direction ofthat cell.

Black bars represent responsesto stimuli in the null direction of thatcell.

As correlation increases,the distance between thedistributions increases.

The pair of spike response distributionsat each correlation was used to find theideal observer response of cell, whichyields a proportion correct at each levelof correlation …

Histograms of one cell’s responses topreferred and null directions

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Neurometric Result

0% correlation 50% correlation 100% correlation

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Comparison of psychometric and neurometric results

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Implication

• The performance of a monkey and theperformance that would be obtained if thatmonkey relied on the output of a single cell aresimilar.

• In essence, a single cell effectively ‘performs’ aswell as the monkey.

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Correlation and causality

• Just because, when presented with a moving stimulus, aparticular cell fires, or an area lights up in a PET scan,can we conclude that this particular neuronal activity isresponsible for motion perception?

• Do the responses of cells in any given area causemonkey’s performance?

• Salzman, Murasugi, Britten, and Newsome (1992)designed an elegant experiment which shed light on thisissue…

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• Monkey performs sameexperiment as above.

• A selected cluster of neuronsin MT with the samepreferred directions isstimulated with electricalpulses.

• Monkey’s behaviour ismonitored to see ifstimulating neurons affectsresponses of monkey.

Stimulating motion detection cells can affect performance

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Stimulation increases proportion of trials on which dots are perceived to move in preferred direction. Stimulation acts as a proxy for increased dot correlation (like adding 7%-20% to dot correlation).

Effect of neural stimulation on monkey performance

No stimulation

stimulation

Null direction 0% Preferred Direction

Effect of stimulation(pushes curve to left)

Stimulation

26Null direction 0% Preferred Direction

Monkey J Monkey E

Monkey JMonkey E

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Conclusions

• These experiments demonstrated, in a live animal, arelationship between neurons and behaviour.

• Strong evidence to suggest that the responses of cells inarea V5 correlate with the subjective perception ofmotion in the monkey, and that cells in V5 causemotion perception.

• For additional reading on this, see Zeki (1993), “Avision of the brain” (chapter 29).

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Reading• Essential Reading• Frisby JP & Stone JV Chapter 14: Seeing Motion Part 1.• Britten, K. H., Shalden, M. N., Newsome, W. T., & Movshon, J. A. (1992).

The analysis of visual motion: A comparison of neuronal and psychophysicalperformance. Journal of Neuroscience, 12, 4745 - 4765.

• Salzman, C. D., Murasugi, C. M., Britten, K. H., & Newsome, W. T. (1992).Microstimulation in visual area MT: Effects on direction discriminationperformance. Journal of Neuroscience, 12(6) , 2331 - 2355.

• Additional Reading• Farah, M. J. (2000). The Cognitive Neuroscience of Vision. Oxford, UK: Blackwells

Publishers Ltd.• Newsome, W. T., & Pare, E. B. (1988). A selective impairment of motion

perception following lesions of the middle temporal area (MT). Journal ofNeuroscience, 8, 2201 - 2211.

• Schenk, T., & Zihl, J. (1997). Visual motion perception after brain damage. I.Deficits in global motion perception. Neuropsychologia, 35, 1289 - 1297.

• Zeki, S. M. (1993). A vision of the brain.• Zeki, S. M., Watson, J. D. G., Lueck, C. J., Friston, K., Kennard, C., &

Frackowiak, R. S. J. (1991). A direct demonstration of functionalspecialization in human visual cortex. Journal of Neuroscience, 11, 641 - 649.