The comparison of separated objects and the coding of visual information

1. The comparison of separated objectsand the coding of visual information M.V. Danilova (I.P.Pavlov Institute of Physiology RAS, St. Petersburg, Russia) J.D. Mollon (Department of Experimental Psychology, University of Cambridge, UK)

3. Long-range comparisons Local comparisons vs How precisely can the human observer compare stimuli that are well separated in the visual field? What are the mechanisms that underlie such comparisons? Why we ask this question? – Because there are both physiological and psychophysical evidence, that different mechanisms are involved in comparisons of nearby and separated objects.

4. Local comparisons can be performed by hard-wired mechanisms extracting local differences in a stimulus attribute and operating over a limited range. Such mechanisms are found for several stimulus attributes: • luminance-opponent ganglion cells (Kuffler 1953; Kaplan et al 1988); • movement detectors (Barlow Levick 1965); • orientation discontinuity detectors (Knierim Van Essen 1992; Sillito et al 1995); • detectors of angles and junctions (Shevelev et al., 1991); • detectors of boundaries between static and moving noise (Hammond Mackay 1975)

5. Local comparisons orientation discontinuity detectors: Sillito Grieve Jones Cudeiro Davis Nature 1995 378 492-496 The orientation discontinuity detectors are the cells in V1 whose receptive fields have classical and non-classical zones. When the centre of the receptive field is stimulated with a grating of one orientation, and the area outside the classical receptive field is stimulated with another orientation, then the cell fires more strongly, especially when the two orientations are perpendicular. Such cells can detect changes in orientation of two adjacent areas.

6. Local comparisons angle and cross detectors: Shevelev Novikova Lazareva Tikhomirov Sharaev Neuroscience 1995 69 51-57 The same cell in V1 responds in a different way to a bar of optimal orientation (open circles), to the right-angled cross (filled circles in b) and to a corner (filled circles in c). Such cells also can detect difference in orientation of two bars, or irregularities in orientation of two adjacent areas.

7. A psychophysical task of texture discrimination is likely based on orientation discontinuity detectors if two texture areas differ in orientation and the task is to detect the presence of the difference. Local comparisons psychophysics physiology orientation discontinuity detectors: Sillito Grieve Jones Cudeiro Davis Nature 1995 378 492-496 The cells are symmetrical to the orientation difference between the central and surrounding areas, so they can not discriminate the direction of difference in orientation

8. Local comparisons are likely to be based on hard-wired mechanisms extracting local differences in a stimulus attribute if the two stimuli fall on to the same or adjacent retinal locations. In this case we can talk about single units, or single neurons, which provide differential information. At some level of the brain the decision can be based directly on this difference signal.

9. All current models of vision start from processing units early in the visual system. These processing units are either orientation and spatial frequency selective cells, or they are cells extracting differences in colour signals from different kind of cones. Though long-range lateral connections in the visual cortex extend for several millimetres, they have limited extent and become systematically sparser with distance. Long-range comparisons?

10. If the two objects are separated by several degrees of the visual angle, if the psychophysical task is to detect the direction of difference (redder or greener, tilted more clockwise, wider or thiner, etc, and if such comparison is based on signals coming from local mechanisms, then psychophysical performance should deteriorate with increasing separation between the two objects. Long-range comparisons?

11. In psychophysical experiments we studied how the precision of comparison depends on spatial separation between the two patches for several stimulus attributes – spatial frequency, orientation, contrast, colour. In all cases two simultaneous stimuli were briefly presented to the observer. But before we can talk about comparison, we have to be sure that the observer do perform an active comparison of the two stimuli. Another strategy that can be applied is to compare only one of the stimuli to an internal template which develops as a mean value of all stimulus values presented during the experiment. In this case a comparison can be done by attending to only one stimulus. To prevent the observers from using a template, a special procedure was developed.

12. separation fixation point imaginary circle One temporal interval, duration 100 msec containing two simultaneous Gabor patches The centres of the Gabor patches always lay on an imaginary circle 5 deg radius; their location was chosen randomly from trial to trial The referent stimulus could be either more or less clockwise; two randomly interleaved staircases measured simultaneously ascending and descending thresholds; The observer did not know in advance which was the referent stimulus and did not know the direction in which the test stimulus differed from the referent which could have one of 25 closely spaced values. Separation varied in separate blocks on the same experimental day; next two slides show examples of the stimuli used Methods

15. Spatial frequency discrimination: thresholds Stimuli: 25 referent frequencies centered at 2 cyc/deg spaced by 1%; orientation was vertical; contrast was 0.3 The observers’ task was to report whether the most clockwise patch (marked by a thin line) is of higher or lower spatial frequency than the other one Results For separations up to 10 deg there is little or no change in the precision with which subjects discriminated the spatial frequency of separated patches

16. Spatial frequency discrimination: response times Subjects were not required to respond as quickly as possible, but the program recorded the interval between the stimulus offset and the moment when the subject pressed a button on the response box For spatial frequency only one subject IB gained precision of judgment at large separations by an increase in processing time

17. Contrast discrimination: thresholds Stimuli: 25 referent contrasts centered at 0.3 spaced by 1%; orientation was vertical; spatial frequeny was 0.3 The observers’ task was to report whether the most clockwise patch (marked by a thin line) is of higher or lower contrast than the other one Results For separations up to 10 deg there is little or no change in the precision with which subjects discriminated the contrast of separated patches

18. Contrast discrimination: response times For contrast all subjects did not gain precision of judgment at large separations by an increase in processing time

19. Orientation discrimination: thresholds Stimuli: 25 referent orientations centered at 45 degrees clockwise spaced by 1 deg; Spatial frequency was 2 c/deg; contrast was 0.3 The observers’ task was to report whether the most clockwise patch (marked by a thin line) was rotated clockwise or anticlockwise compared to the other one Results For separations up to 10 deg there are some subjects who show no change in the precision in discriminating orientation of separated patches, but there are subjects whose performance deteriorated with increasing separation

20. Orientation discrimination: response times The trend is different for different subjects and a given subject’s response times to some degree reflect the trend in his/her thresholds: MD and JM become slower at large separations, whereas IB becomes faster and NL does not show a significant upwards or downwards trend.

21. orientation discrimination thresholds Same-different judgements may rely on local orientation discontinuity detectors, and such performance deteriorates with spatial separation. Direction of tilt judgements use another mechanisms and show little change with spatial separation response times

22. An analogue of MacLeod-Boynton space was created using 10-deg fundamentals because we present our stimuli at eccentricity 5 deg from fixation

23. METHODS The L/(L+M) coordinate was fixed at 0.7 and the S/(L+M) coordinate was varied to measured the discrimination threshold. 25 referent S-values were spaced by 1% steps symmetrically around the S-value of equal-energy white. Stimulus duration was 100 ms. The spatial separation varied in different block of trials from 2 to 10 degrees.

24. imaginary circle radius 5 deg separation fixation point

26. RESULTS The three subjects show decrease in thresholds when the patches are separated even by a very small gap of 0.15 degrees (second point on the graphs). This is the gap-effect (Boynton, Hayhoe, MacLeod. Optica Acta 1977 24 159-177), which is thought to be characteristic of discriminations based on short-wave-cone signals. Apart from the gap-effect, colour discrimination was little affected by the spatial separation between the two target patches.

27. When the observer is required to report the direction of difference in orientation, contrast, spatial frequency or colour, the curves relating thresholds and spatial separation are rather flat. This led us to conclude that comparisons of well-separated objects should be performed at a more central level than local discontinuity detectors and we question models that are dependent on long-range interactions between local analysers in the primary visual cortex. In this case processing of information to the higher levels of the visual system where the comparison takes place encounters encoding problem.

28. A cerebral bus? We favour an alternative hypothesis that is not often considered in current neuroscience. Beyond the cortical stage where local stimulus features are represented by the activity of single units, there may be a radical transformation, so that stimulus attributes are represented in an abstract code that can run on a cerebral bus in the way that a computer bus or an Internet link carries different messages for different destinations. The essential feature of the Internet is that it eliminates the need for a dedicated cable between any particular pair of computers that need to communicate. Communication is by packets of data that carry with them, in an agreed code, the identity of the sender and that of the addressee. In the brain, as on the Internet, the information available at a local site may be required from time to time at many different sites: it is possible that the brain too has side-stepped the proliferation of dedicated lines for some tasks and has evolved a means of representing stimulus attributes – as well as objects, words and concepts – that does not depend on the activity of single cells.