Talk:Vernier acuity

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JS

  • Subjects notice different aspects of the stimulus, depending on the task. For instance they don't notice that the line is longer if they're focussing on the task of detecting whether the line is shifted to the right or to the left. Why suppose that subjects' perceptions differ correspondingly? Might their perception not remain constant while what they notice differs?

AG

  • I would like to try and hold to the line that 'noticing' is what matters about perception. If I can't notice the difference between the short line and the long line (e.g. as determined by a forced choice response) then my perception of the two is the same (it seems to me, until persuaded otherwise). You point out that other evidence, e.g. from priming can demonstrate that the visual system can record the difference between the short line and the long line (it is, after all, a supra-threshold difference in length when the task is to detect line length) and this trace can emerge when probed by other tasks. That is fine, it demonstrates that the visual system recorded the length difference (i.e $\mathbf{s}$ is different for the two line lengths but, for a given line length $\mathbf{s}$ remains the same for the two tasks: the vernier task, where line length is ignored, and the line length task, where the length difference is easily picked up). But to say that the visual system (or more generally the sensory component $\mathbf{s}$ of the sensory+motivational context $\mathbf{r}$) has a record of the length difference does not mean (it seems to me) that that length difference is perceived. What changes $\mathbf{r}$ between the two tasks is the motivational component, $\mathbf{t}$. A demonstration that in a third task, a priming task, it can be shown that $\mathbf{s}$ contains a record of the line length difference seems fine, but this is not a demonstration that subject's perception differs with line length in the catch trial for the vernier detection task. Your question relates to one stimulus, one perception and two different things noticed (e.g. the long line in the vernier and line-length tasks). In this very limited case, I think I would be happy to say that perceiving and noticing amount to the same thing. I can imagine other situations with more loops in the spiral where the distinction between noticing and perceiving might become greater. If I walked into a room twice, made ten saccades and walked out, I might claim that my perception of the room was the same on each occasion but, because I fixated on different things, I might have noticed different things and subsequent tests might show that this was the case. But for the single flashed presentation of a line, the point here is that the task makes a big difference to the perception of the stimulus and that difference is explained here by the motivational component of the current context.