ARVO96_Poster

Visual Search for Flicker: High Temporal Frequency Targets Capture Attention

Robert.F. Dougherty, Alison Smith, Mark R. Verardo, Melanie J. Mayer
University of California, Santa Cruz, CA.

Purpose

Flicker and attention
- Flickering lights can be quite powerful at attracting visual attention
  - emergency vehicles and road hazards
  - control panel warning indicators
- Our aim is to study search for a target defined by flicker rate.
We ask:
- Can search performance be explained by the target-distractor discriminability in a simple search-for-flicker task?
- How easily the target is discriminated from the distractors is an important factor in the visual search task (Bergen & Julesz 1983; Duncan & Humphreys 1989; Verghese & Nakayama 1994)
  - Do spatial precues facilitate performance in this simple search task?
  - Does rapid flicker capture attention, as expected given the capture of attention by abrupt stimulus onsets? (Yantis & Jonides 1990; Krumhansl 1982)

Flicker sensitivity

Flicker sensitivity is a measure of how well the visual system responds to local change in luminance over time.

This function is often referred to as a de Lange function (de Lange, 1958).
Because sensitivity varies as a function of flicker rate, the perceived modulation depth also varies with flicker rate. This perceived flicker contrast was controlled by setting the contrast of each flickering Gabor patch to twice the subject's detection threshold for that flicker rate (Waugh & Hess, 1994).

Methods

Apparatus
- Apple High Resolution Monochrome monitor with a white phosphor (P4)
- 30 cd/m2 mean luminance
- Macintosh Quadra 840 AV, custom software (using Pelli's VideoToolbox) & Apple 8-bit video board
Procedure
- Monocular viewing
- 2-interval, forced-choice paradigm
- 2, 4 or 8 possible target locations cued 1 second before stimulus presentation (target always appeared at one of these cued locations)
- Non-speeded response
- Stimulus duration fixed for each trial, varied across trials with a staircase
- Trials blocked & counterbalanced for target flicker rate and number of cues
- Feedback presented after each trial
- All observers well practiced

Stimulus

Precues
- 250 millisecond duration
- All stimulus locations indicated by a cross
- Cues balanced to minimize eye movement (e.g., with two cues, locations at opposite sides of the array would be cued)
Stimulus array
- 8 1 cycle/deg Gabor patches counterphase modulated at 2, 4, 8 or 17Hz
- Gabors subtend 0.6° at half-height
- Onset smoothed by ramping up the contrast for the first 75 ms
- Contrast set to twice detection threshold
- Gabors presented at 3-5° eccentricity, concentric around foveal fixation mark (the eccentricity was randomly jittered to break up the perfect circle)
- Temporal phase: target fixed, distractor phases randomized
Mask
- 250 milliseconds of dynamic noise

Data Analysis

Critical duration
- The stimulus duration necessary for criterion performance (82%) was determined by fitting a Weibull function with a maximum likelihood procedure (Watson, 1979)
- The bootstrap method (a Monte Carlo variance estimation procedure) was used to estimate the standard errors of the critical duration parameter estimates (Foster & Bischof, 1991)
Discriminability index
- An index of flicker discriminability was estimated from the suprathreshold foveal flicker discrimination data of Waugh & Hess (1994; from their figure 3)

Results

Cueing effects for two observers. Six target/distractor flicker pairs were tested. The legend indicates the target flicker rate (first value) and the distractor flicker rate (second value). The discriminability index for the pair is indicated in parentheses.

Do spatial precues facilitate search for flicker?
- Yes and No. (see cueing effects graphs)
- Precues facilitate search for some target-distractor flicker pairs
  - Generally, the less discriminable conditions show more of a cueing effect than the more easily discriminated target-distractor flicker pairs
  - This confirms our previous finding (Dougherty, Verardo & Mayer 1995), even while removing the confound of set size and stimulus density

Discriminability and critical duration. Critical duration as a function of discriminability in log-log coordinates for two observers.

Can discriminability explain the search time effects?
- No. (see discriminability vs. critical duration graphs)
- Discriminability is not the whole story
  - If discriminability did account for the processing time effects, we would expect a smooth, simple relationship between it and critical duration in the figures above
  - The discontinuity suggests that something else is going on for some of these conditions
  - The aberrant points all involve 17 Hz targets with lower than expected critical durations- perhaps these rapidly flickering gabors are capturing attention?

Equal discriminability conditions. Two conditions with the same discriminability index are shown for two observers. (Error bars = 95% confidence interval.)

Does high frequency flicker capture attention?
- Yes, rapid flicker captures attention
  - In this experiment, the discriminability is exactly the same for both conditions (because the same two flicker rates are used in both conditions)
  - The condition with the rapidly flickering target has significantly shorter critical durations than the one with the rapidly flickering distractors
  - This is consistent with the attentional capture effects produced by abrupt stimulus onsets (Yantis & Jonides 1990; Krumhansl 1982)

Conclusions

Present findings
- Precueing facilitates low discriminability searches
- Attention may be difficult to allocate to 4 separate locations (4 cues are no better than 8 cues for one observer)
- Stimulus discriminability is still useful for explaining visual search for different flicker rates
- However, rapid flicker captures attention, allowing shorter search times for all three relevant set sizes
Future directions
- What flicker rates are optimal for capturing attention?
- How distracting are these rapidly flickering regions?
  - By making flicker irrelevant to the task (e.g. search for orientation with a flickering distractor), we can determine if the flicker is necessarily disruptive

References

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