X The Troxler effect: the fading of a large stimulus with blurred edges presented in the peripheral visual field

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)

Part One Temporal Acuity: The critical flicker frequency (CFF)

Measure CFF using an episcotister (a rotating sectored disk used to produce square- wave flickering stimuli)

How bright does a fused flickering light appear? To convert duty cycle to f, divide the first number by the sum of the two numbers: 1:1 means f=0.5

If a square-wave flickering light has a duty cycle of 4:1, what is f?

To convert duty cycle to f, divide the first number by the sum of the two numbers: 4:1 would be 4/(4+1) so f=0.8

How bright does a flickering light appear? At flicker rates slightly below the CFF, brightness is enhanced beyond the mean luminance of the flicker (the Brücke-Bartley phenomenon) This is related to the Broca-Sulzer effect described later in the chapter

The neural basis of the CFF is the modulation of firing rates of retinal neurons (ganglion cells)

Courtesy of Dr. Tim Kraft Cone flicker response (pig). Contrast 0.49; mean light level 48,300 photon/ square micron

Rat ganglion cell responses showing CFF

In order to see a light as flickering 1.The flicker rate must be above the CFF 2.The Troxler effect must occur 3.Retinal neurons must be able to respond with gaps in their firing pattern 4.All of the other answers are correct

How does this measure of temporal acuity (CFF) change under different conditions (changes in the stimulus dimensions listed in Chapter 1)? First: stimulus luminance (intensity)

Note: if the luminance of the stimulus increases by one log unit, so does the retinal illuminance

demo 1)Find CFF 2)Raise intensity (luminance) by 1 log unit. The more intense stimulus is below CFF (flicker is seen). 3)Have to increase the flicker rate to again find CFF

0 o 3 o 10 o 35 o 65 o 85 o

Retinal Illuminance (td) The CFF is highest in the midperipheral retina at high luminance, but nearly constant across the retina at low luminance. This is why you can see flicker on some PC monitors if you look slightly to the side

CFF increases 1.In direct proportion to the log of the stimulus luminance 2.In the periphery at all luminance levels 3.In response to the Brücke-Bartley phenomenon 4.None of the above

Second: area (size) Demo, since I haven’t found a good figure showing this relationship CFF = k logA + b Where k and b are constants and A is the area of the flickering stimulus

Demo – Granit-Harper 1)Find CFF 2)Increase stimulus area by 1 log unit. The more intense stimulus is below CFF (flicker is seen). 3)Have to increase the flicker rate to again find CFF.

Chapter 7 – Temporal Factors in Vision Main points so far: 1)CFF is a measure of temporal acuity – analogous to VA(how small a temporal interval can you detect – in time)? 2)CFF increases linearly with log stimulus luminance (Ferry-Porter Law) 3)CFF increases linearly with log stimulus area (Granit-Harper)

You will not be responsible for the material starting on page 188, “flicker sensitivity increases….” and including all of page 189 and 190 (Figs. 7.7 and 7.8). You will be responsible for material starting again on page 191, “Temporal Contrast Sensitivity”

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)

Contrast, modulation and amplitude The contrast of a temporal sine wave is defined the same way as the contrast of a spatial sine wave grating: Contrast = (L max  L min )/( L max + L min ) In Figure 7-1, L max is 300, L min is 100, so contrast = (200)/(400) = 0.5 Another term,modulation (abbreviated asm),is sometimes used for sine-wave flicker, and may be used interchangeably with contrast. As illustrated in Figure 7-1, L max is the maximum luminance of the flicker, and L min is the minimum luminance. L max and L min are symmetrically arranged around the mean or average luminance, defined as: Mean Luminance = L m = ( L max + L min )/2 Hence, contrast or modulation can also be expressed as: Contrast =m = (L max  L m )/ L m In addition, L max - L m is also called the amplitude of the wave, and, therefore, Contrast = modulation = amplitude /L m Referring again to the sine wave at the bottom of Figure 7-1, the mean luminance is 200 units, the amplitude is 100, and the contrast (modulation) therefore is 0.5. As was the case for spatial sine-wave gratings, contrast sensitivity is defined as the inverse of the threshold contrast.

This is like Figure 6.9 in the spatial domain

Temporal CSF Demo

Change in the temporal CSF with luminance: As luminance decreases,  the peak contrast sensitivity becomes lower  the cutoff high temporal frequency decreases (Ferry-Porter law)  peak contrast sensitivity occurs at lower temporal frequency  the low temporal frequency rolloff disappears

The temporal contrast sensitivity function 1.Is the boundary between contrasts you can see and ones you cannot see 2.Has a peak contrast at around 1 Hz at high mean luminance 3.Is a measure of temporal acuity 4.Becomes more bandpass as the mean luminance is decreased

The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at some temporal frequencies

The delayed arrival of the surround signal, relative to the center signal can cause the surround to add with the center at mid-range temporal frequencies

This is like Figure 6.9 in the spatial domain

1.Artificially increased IOP produces reduced temporal CSF (but no effect on CFF) 2.Temporal CSF is reduced with glaucoma and ocular hypertension Glaucoma - Frequency-doubling perimeter measures contrast threshold for 0.25 c/deg grating flickering at 25 Hz (mediated by MY [nonlinear magno] cells?) 3.Eyes at risk for exudative (wet) AMD show reduced sensitivity at Hz (5 Hz & 10 Hz alone discriminate from healthy eyes) Importance? Early diagnosis can lead to earlier treatments The temporal CSF is a useful measure for diagnosing retinal disorders

The low temporal frequency rolloff of the temporal CSF 1.Is really a “mid-temporal frequency enhancement produced by the longer latency of the receptive field surround 2.Becomes more prominent at low mean luminance levels 3.Helps create Mach bands 4.Is related to the cutoff high temporal frequency

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation (Bloch’s Law & Broca-Sulzer) Masking Motion Detection (Real and Apparent)

Fig. 2.5

Bloch’s Law holds for durations shorter than the critical duration There is a constant # of quanta in a threshold flash as L decreases

Temporal Summation and Bloch's Law When a brief flash is used to determine the threshold intensity, the visual system does not distinguish the “temporal shape” of the flash if the flash duration is less than the “critical duration” Part A – threshold measures

Bloch’s Law holds “Holds” means that Bloch’s Law accounts for the threshold values Two ways to show Bloch’s Law: L x t = C

But I will not hold you responsible for this section Bottom of 198 & top of 199

Flashes of various durations shorter than the critical duration all have the same temporal frequency spectrum. Flashes longer than the critical duration contain less contrast at intermediate temporal frequencies, after filtering through the temporal CSF and are therefore less visible. Thus, more quanta are need to be added to bring them up to threshold.

For flash durations less than the critical duration, Bloch’s Law holds and 1.The flash cannot be seen when it is above threshold 2.The number of quanta in a threshold flash is the same for different flash durations 3.L x C = t 4.None of the above

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation (Bloch’s Law & Broca-Sulzer) Masking Motion Detection (Real and Apparent)

Part B – above-threshold brightness Broca

Neural Explanation Intense stimuli produce photoreceptor overshoot This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells Brightness is related to the firing rate of the cells (spikes/second) For long flashes, the firing rate after the initial burst signals the brightness For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.

Neural explanation of the Broca-Sulzer effect Note: the photoreceptor membrane potentials are upside down (negative is up on the graph) to demonstrate the similarity in shape to the Broca- Sulzer effect.

Neural Explanation Intense stimuli produce photoreceptor overshoot This produces (via the bipolar cells) an initial burst of action potentials in the ganglion cells Brightness is related to the firing rate of the cells (spikes/second) For long flashes, the firing rate after the initial burst signals the brightness For brief flashes, only the initial burst occurs, so the only information the neurons in central structures can use is a high firing rate, which makes the flash appear brighter than when it is long.

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking (Temporal interactions between visual stimuli) Motion Detection (Real and Apparent)

Masking Forward masking Backward masking Backward Masking might remind you of Early Dark Adaptation

Test flash

Masking flash

Masking Forward masking Backward masking Backward Masking might remind you of Early Dark Adaptation

Simultaneous and forward masking are signal detection problems

Backward masking may be explained by the response latency and duration of the test flash

Masking effects may occur when the test and mask are spatially separated metacontrast (backwards) and paracontrast (forwards) are masking in which the test flash and masking flash do not overlap spatially on the retina This suggests that the same cells must be stimulated by the edges of both stimuli to obtain metacontrast

When the gap between the stimuli becomes large enough, different populations of retinal neurons are stimulated by the test and masking flashes. Any masking has to occur upstream in the visual pathway, where receptive fields get larger

Masking Masking is any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus. Metacontrast (backwards masking with physically- separated stimuli) and paracontrast (forward masking with physically separated stimuli) Dichoptic masking – masking where the two stimuli are presented to different eyes

Cannot occur until inputs from the two eyes meet at a binocular cell in V1 or later

Saccadic suppression Saccadic suppression is defined as a reduction in sensitivity to visual stimuli that occurs before, during and after a saccade (Look at your eyes in a mirror and try to see them move when you make a saccade)

Eyes moving But you can see the strobe lights atop Red Mountain if you time your saccade just right

Masking includes 1.any situation in which the detection of a visual stimulus is reduced by another stimulus presented before, during, or after the target stimulus 2.Paracontrast 3.Dichoptic masking 4.All of the above

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)

Motion is a continuous change in an object’s location as a function of time Three reasons motion detection is important: detect moving objects against a background (see edges) detect own motion through the environment determine 3-D shape (crudely)

Demo – shape from motion (If you can’t see the edges, you can’t see the object)

Real Motion: Motion involve an image changing its location on the retina

Contrast with smooth pursuit (moving the eyes smoothly). This prevents the image from changing its location on the retina. We are not studying smooth pursuit.

There is an upper limit to our ability to see motion – stimuli can be moving “too fast to see” It turns out that the reason is that rapidly moving images have a temporal frequency that is too high for our visual system to detect (frequency is above the temporal high- frequency cutoff). To understand this – need to look at movement from the point of view of an individual retinal neuron.

From the viewpoint of any one cell in the retina, motion is a change in luminance that occurs at a rate that depends on the speed with which the object moves and on the spatial frequency composition of the object A high spatial frequency grating moving at constant velocity (degrees per second) has a faster temporal frequency than a lower spatial frequency moving at the same velocity

This grating moved one full cycle

Motion involves interactions of both spatial frequency and temporal frequency Now, a lower spatial frequency (about half of the first one) moving at the same velocity (degrees per second). It has lower temporal frequency (cycles per second) at a given spot

This grating moved about ½ cycle. Measured at the green dot (symbolizing a receptive field), it has a temporal frequency (flicker rate) about half of the higher spatial frequency Can determine the temporal frequency of a drifting grating by multiplying its spatial frequency times its velocity in degrees per second A 3 cycle/deg grating moving 10 deg/sec has a temporal frequency of 30 Hz; 30 cycle/deg =300 Hz

As object velocity increases, spatial CSF shifts to lower spatial frequencies; temporal CSF remains constant

How fast a velocity can you see moving? The limiting factor in motion detection is the temporal resolution of the visual system. If you present a very, very low spatial frequency (and high contrast) can see motion of several thousand degrees per second

The ability to see rapidly-moving (high velocity) objects 1.Is limited by the temporal frequency 2.Occurs only in the visual cortex 3.Is set by the velocity of the objects 4.Cannot be measured

Apparent Motion: Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously. Phi phenomenon

Apparent Motion: Apparent motion is the perception of real motion that can be produced when a stimulus is presented discontinuously. The “rules” for producing apparent motion are the same as for real motion: the optimal stimulus duration and spacing is the same as would occur if a real object moved.

To produce optimal apparent motion of 10 degrees per second, need each spot to be about 25’ apart and be on for about 35 msec. A real object, traveling at 10 degrees per second would move 21’ in the same time

In order to make apparent motion look like real motion 1.You have to “fool” some of the neurons all of the time 2.You need a string of lights 3.You need real motion 4.You need to present the stimuli with the same separation and duration as would occur with real motion

Detection of motion and sensitivity to direction of motion is achieved in hierarchic fashion in Areas V1 of the striate and middle temporal region of the cortex

Newsome and colleagues sampled the activity of neurons in area MT Each cell has a receptive field that responded to motion in some location in the visual field (some retinal location). Each neuron was direction selective; it had an optimal direction (most spikes per second) and a null direction (fewer spikes per second).

Stimuli with a range of correlation of the motion of the spots were used to determine threshold amount of correlation for the monkey, and also the threshold for neurons in the monkey’s area MT (in a two- alternative, forced-choice situation).

Using signal detection theory, a “neurometric” function could be produced for each neuron and compared with the monkey’s psychometric function

Real & apparent motion seem to be detected by neurons in the parietal (MT) “stream” The psychometric function for the monkey was matched well by direction- selective neurons in area MT. Monkey more sensitive than the neuron Neuron more sensitive than the monkey

The monkeys’ “neurometric function” 1.Did not match the psychometric function 2.Could not be accurately estimated 3.Closely matched the psychometric function 4.None of the above

Adapting to one direction of motion can produce a motion aftereffect when the movements stops (the “waterfall illusion”) May be due to neurons in MT Waterfall Illusion

Five Parts to this Chapter Temporal Acuity (critical flicker frequency [CFF]) The Temporal Contrast Sensitivity Function Temporal Summation Masking Motion Detection (Real and Apparent)