It can be helpful to think of an object (or visual stimulus) as having a single value along each of several property dimensions. For example, a line’s orientation could be anywhere between −90 and +90 degrees with respect to vertical. And an object’s colour could be anywhere between violet (shortest visible wavelength) and red (longest visible wavelength). The general rule that describes perceptual after-effects is that adapting to some value along a particular dimension (say +20 degrees from vertical) makes a different value (say 0 degrees) appear even more different (say −5 degrees). For this reason, these phenomena are sometimes called negative after-effects. The after-effect is in the opposite direction (along the stimulus dimension) away from the adapting stimulus, rather than moving the perceived value towards that of the adapting stimulus. What do these effects tell us about how perceptual systems encode information about the environment? The existence and properties of channels One implication of afte -effects is that different features, or dimensions, of a stimulus are dealt with separately. Each dimension is, in turn, coded by a number of separate mechanisms, often called channels, which respond selectively to stimuli of different values along that particular dimension. Each channel responds in a graded fashion to a small range of neighbouring values of the stimulus dimension. So several channels respond to any given stimulus, but to differing extents. The channel that most closely processes (i.e. is most selective for) the stimulus will give the greatest output, channels selective for nearby stimuli will give a lesser output, and so on. For example, different channels may selectively code for different angles of orientation of visual stimuli, from horizontal round to vertical. This enables us to give a simple explanation of after-effects, illustrated in this chapter using the tilt after-effect (Blakemore, 1973). Perception depends not on the output of any single channel, but on a combination of the outp ts of all the active channels. This is because a given level of activity in any single channel might be caused by a weak (say, low contrast) stimulus of its optimal type (such as a vertical line for a channel that responds best to vertical lines) or an intense (high contrast) stimulus away from the optimal (such as a line tilted 20 degrees). So the output of a single channel on its own is ambiguous. For the sake of simplicity, we will look at the relationship between just five channels, although in practice there are many more. In panel A, each bell shaped curve (‘tuning curve’) represents the activity in one channel produced by lines of different orientations. One channel responds most strongly to vertical lines (the channel whose tuning curve is centered on 0 degrees), and progressively less strongly to stimuli further and further from that optimal orientation of line (either clockwise or anti-clockwise). Another channel has the same degree of selectivity but responds best to lines tilted to the right by 20 degrees. A third channel is similar but ‘prefers’ (or is ‘tuned’ to) tilt in the opposite direction from vertical (−20 degrees). The orientations over which these latter two channels respond overlap, so they respond weakly but equally to zero tilt (vertical stimuli), as shown in panel B. Finally, we include two outermost channels, which respond best to 40 degrees (+40 deg) clockwise and 40 degrees anti-clockwise (−40 deg.). These two channels do not respond at all to vertical lines. This system of channels can signal orientations which do not correspond to the preferred orientation of any single channel. Panel C shows the pattern of activation produced by a line tilted 5 degrees anticlockwise. Compared with activity produced by a vertical line, activity in the −20 degree channel has increased and that in the other two channels has decreased. How is the information from all these channels combined when a visual stimulus is presented? There is likely to b a process that combines the activities across all channels, weighted according to the level of activity in each channel. Such a process finds the ‘centre of gravity’ of the distribution of activity. The centre of gravity (in statistical terms, the weighted mean) corresponds to the perceived orientation of the stimulus. The tilt after-effect During prolonged stimulation, the activity in the stimulated channels falls – in other words, channels ‘adapt’. This fall is proportional to the amount of activity, so adaptation is greatest in the most active channels. After the stimulus is removed, recovery occurs slowly. We can see the effects of adaptation by presenting test, or ‘probe’, stimuli in the period shortly after the adapting stimulus has been removed. For example, think back to the waterfall illusion: when you gaze at a waterfall and then transfer your gaze to a point on the banks of the waterfall, you notice an apparent dramatic upward movement of the banks. So we can explain the tilt after-effect as follows. Initially, all channels have equal sensitivity. During presentation of a vertical stimulus, the distribution of active channels is symmetrical about zero, so the perceived orientation corresponds to the actual stimulus orientation – i.e. vertical. A stimulus that falls between the optimal values of two channels is also seen veridically (that is, true to its actual orientation) by taking the centre of gravity of the activity pattern; this is how we see, for example, a small degree of tilt away from vertical. With stimuli tilted 20 degrees clockwise, the active channels are also symmetrically distributed and have a centre of gravity at 20 degrees, so perception is again veridical. But during a prolonged presentation of such a stimulus (for, say, 60 seconds), the 20 degree channel adapts and its sensitivity declines. The reduction in each channel’s sensitivity is proportional to the amount that it is excited by the timulus, so the 0 degree and 40 degree channels are also adapted and have become less sensitive due to the presentation of this stimulus tilted 20 degrees clockwise, although to a smaller extent than the 20 degree channel. (The two channels that respond best to anti-clockwise tilts are not adapted at all.) The effects on sensitivity in the channel system of adapting to +20 deg stimulus are shown in panel E, figure 8.3. Sensitivity is reduced most in the +20 deg channel, and to a lesser but equal extent in the 0 and +40 deg channels. What happens when we present a test stimulus whose tilt is zero. The −20 degree channel will give a small output, as normal, because the stimulus is away from the channel’s optimal orientation, although within the range of tilts to which it is sensitive. But the output of the +20 degree channel will be even smaller, not only because the stimulus is not optimal for the channel, but also because the channel’s sensitivity has been reduced by the prior adaptation to a 20 degree stimulus. So the −20 degree channel will clearly be more active than the +20 degree channel, although its normal optimal is equally far from the vertical orientation of the stimulus. The distribution of activity across channels will therefore be asymmetrical, with its mean shifted towards negative tilts. So, after adaptation to a +20 deg stimulus, the pattern of activity in the channel system produced by a vertical test stimulus will be identical to that produced before adaptation by a −5 deg stimulus. So the observer’s percept is of a tilt at 5 degrees to the left. Finally, as the channel’s sensitivities return to normal after adaptation, so the apparent orientation of the test bar changes back to vertical. This general idea can explain other after-effects too, such as those for luminance and colour, for texture, pitch, and so on.
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Monday, December 20, 2010
EXPLAINING AFTER-EFFECTS
It can be helpful to think of an object (or visual stimulus) as having a single value along each of several property dimensions. For example, a line’s orientation could be anywhere between −90 and +90 degrees with respect to vertical. And an object’s colour could be anywhere between violet (shortest visible wavelength) and red (longest visible wavelength). The general rule that describes perceptual after-effects is that adapting to some value along a particular dimension (say +20 degrees from vertical) makes a different value (say 0 degrees) appear even more different (say −5 degrees). For this reason, these phenomena are sometimes called negative after-effects. The after-effect is in the opposite direction (along the stimulus dimension) away from the adapting stimulus, rather than moving the perceived value towards that of the adapting stimulus. What do these effects tell us about how perceptual systems encode information about the environment? The existence and properties of channels One implication of afte -effects is that different features, or dimensions, of a stimulus are dealt with separately. Each dimension is, in turn, coded by a number of separate mechanisms, often called channels, which respond selectively to stimuli of different values along that particular dimension. Each channel responds in a graded fashion to a small range of neighbouring values of the stimulus dimension. So several channels respond to any given stimulus, but to differing extents. The channel that most closely processes (i.e. is most selective for) the stimulus will give the greatest output, channels selective for nearby stimuli will give a lesser output, and so on. For example, different channels may selectively code for different angles of orientation of visual stimuli, from horizontal round to vertical. This enables us to give a simple explanation of after-effects, illustrated in this chapter using the tilt after-effect (Blakemore, 1973). Perception depends not on the output of any single channel, but on a combination of the outp ts of all the active channels. This is because a given level of activity in any single channel might be caused by a weak (say, low contrast) stimulus of its optimal type (such as a vertical line for a channel that responds best to vertical lines) or an intense (high contrast) stimulus away from the optimal (such as a line tilted 20 degrees). So the output of a single channel on its own is ambiguous. For the sake of simplicity, we will look at the relationship between just five channels, although in practice there are many more. In panel A, each bell shaped curve (‘tuning curve’) represents the activity in one channel produced by lines of different orientations. One channel responds most strongly to vertical lines (the channel whose tuning curve is centered on 0 degrees), and progressively less strongly to stimuli further and further from that optimal orientation of line (either clockwise or anti-clockwise). Another channel has the same degree of selectivity but responds best to lines tilted to the right by 20 degrees. A third channel is similar but ‘prefers’ (or is ‘tuned’ to) tilt in the opposite direction from vertical (−20 degrees). The orientations over which these latter two channels respond overlap, so they respond weakly but equally to zero tilt (vertical stimuli), as shown in panel B. Finally, we include two outermost channels, which respond best to 40 degrees (+40 deg) clockwise and 40 degrees anti-clockwise (−40 deg.). These two channels do not respond at all to vertical lines. This system of channels can signal orientations which do not correspond to the preferred orientation of any single channel. Panel C shows the pattern of activation produced by a line tilted 5 degrees anticlockwise. Compared with activity produced by a vertical line, activity in the −20 degree channel has increased and that in the other two channels has decreased. How is the information from all these channels combined when a visual stimulus is presented? There is likely to b a process that combines the activities across all channels, weighted according to the level of activity in each channel. Such a process finds the ‘centre of gravity’ of the distribution of activity. The centre of gravity (in statistical terms, the weighted mean) corresponds to the perceived orientation of the stimulus. The tilt after-effect During prolonged stimulation, the activity in the stimulated channels falls – in other words, channels ‘adapt’. This fall is proportional to the amount of activity, so adaptation is greatest in the most active channels. After the stimulus is removed, recovery occurs slowly. We can see the effects of adaptation by presenting test, or ‘probe’, stimuli in the period shortly after the adapting stimulus has been removed. For example, think back to the waterfall illusion: when you gaze at a waterfall and then transfer your gaze to a point on the banks of the waterfall, you notice an apparent dramatic upward movement of the banks. So we can explain the tilt after-effect as follows. Initially, all channels have equal sensitivity. During presentation of a vertical stimulus, the distribution of active channels is symmetrical about zero, so the perceived orientation corresponds to the actual stimulus orientation – i.e. vertical. A stimulus that falls between the optimal values of two channels is also seen veridically (that is, true to its actual orientation) by taking the centre of gravity of the activity pattern; this is how we see, for example, a small degree of tilt away from vertical. With stimuli tilted 20 degrees clockwise, the active channels are also symmetrically distributed and have a centre of gravity at 20 degrees, so perception is again veridical. But during a prolonged presentation of such a stimulus (for, say, 60 seconds), the 20 degree channel adapts and its sensitivity declines. The reduction in each channel’s sensitivity is proportional to the amount that it is excited by the timulus, so the 0 degree and 40 degree channels are also adapted and have become less sensitive due to the presentation of this stimulus tilted 20 degrees clockwise, although to a smaller extent than the 20 degree channel. (The two channels that respond best to anti-clockwise tilts are not adapted at all.) The effects on sensitivity in the channel system of adapting to +20 deg stimulus are shown in panel E, figure 8.3. Sensitivity is reduced most in the +20 deg channel, and to a lesser but equal extent in the 0 and +40 deg channels. What happens when we present a test stimulus whose tilt is zero. The −20 degree channel will give a small output, as normal, because the stimulus is away from the channel’s optimal orientation, although within the range of tilts to which it is sensitive. But the output of the +20 degree channel will be even smaller, not only because the stimulus is not optimal for the channel, but also because the channel’s sensitivity has been reduced by the prior adaptation to a 20 degree stimulus. So the −20 degree channel will clearly be more active than the +20 degree channel, although its normal optimal is equally far from the vertical orientation of the stimulus. The distribution of activity across channels will therefore be asymmetrical, with its mean shifted towards negative tilts. So, after adaptation to a +20 deg stimulus, the pattern of activity in the channel system produced by a vertical test stimulus will be identical to that produced before adaptation by a −5 deg stimulus. So the observer’s percept is of a tilt at 5 degrees to the left. Finally, as the channel’s sensitivities return to normal after adaptation, so the apparent orientation of the test bar changes back to vertical. This general idea can explain other after-effects too, such as those for luminance and colour, for texture, pitch, and so on.
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