Friday, May 30, 2008

Tydgat & Grainger (in press) in JEP:HPP

In this paper, the authors look at perceptual patterns for 5-character strings composed of letters, symbols (i.e., non-alphabetic characters such as @), or a combination. The string was presented for 100 ms, and then masked. As has been previously found, there is no external character advantage for symbols; the first and last symbol were perceived no better than the second and fourth symbols. For letters, they found a strong initial-letter advantage, but in experiments 1-4, there was no final-letter advantage. This pattern held for letter targets in strings of mostly symbols, and for symbols in strings of mostly letters. This shows that the patterns are inherent to the characters themselves, and not driven by different patterns of attention to letter vs. symbol strings.

In experiments 5 and 6, the authors attained a final-letter advantage. Experiments 1-4 used partial report 2AFC, where the two choices appeared directly above and below the target position when the backward mask appeared. Experiment 5 used free report, and experiment 6 used 2AFC, but target position was indicated by surrounding lines and the two choices appeared well below where the stimulus had been. Thus it appears that surrounding letters inhibit the final-letter advantage, but not the initial-letter advantage.

The authors proposed that this asymmetry was due to different receptive-field shapes in the two hemispheres, with RVF/LH fields being circular and LVF/RH fields being elongated along the horizontal axis. As a result, there is more vertical interaction in the RVF/LH and so characters above and below the target have a stronger masking effect, explaining the lack of final-letter advantage when the choices appeared around the target letter.

The authors also noted that experiments 5 and 6 yielded a W-shaped pattern, where accuracy levels were similar for the first, third, and fifth letters, and accuracy levels were lower and equal to each other for the second and fourth letters. The authors claim that this symmetric pattern is inconsistent with the left-to-right serial processing proposed in the SERIOL model.

As for the difference between letters and symbols, the authors suggest that receptive fields are larger for symbols than for letters. Symbol detectors cover the neighboring positions, whereas letter detectors are more narrowly focused. Therefore, a single neighboring symbol has a strong ceiling effect and there is little advantage for having only one neighbor. In contrast, a single neighboring letter has a much weaker effect, and so there is an advantage for having only one neighboring letter, and so the outer letters are more easily perceived.

Before commenting on this article, I would like to say how glad I am to see that Grainger and colleagues are now investigating perceptual patterns. I have been arguing for quite a while that examination of perceptual patterns can tell us a lot about visual specializations for letter string processing, and its good to see experimentation in this area.

Once again, JEP:HPP did not ask me to review this paper, despite the fact that I am probably the leading expert on perceptual patterns for letter strings. So I make my comments here ...

First, about the final-letter advantage. I would say that the reason that surrounding letters affected the final letter but not the initial letter was because they appeared when the final letter was being read out. Due to seriality, the initial letter had already been fully processed at that point, and so was not affected. Thus I suggest that the effect depends on timing and position within the string, not on retinal location.

Now suppose you had an LVF letter at the same retinal location as the letter in position 1 and an RVF letter in the same location as the letter in position 5, but each of those letters appeared as the first or second letter in a string. Under Tydgat and Grainger's account, vertically- surrounding choices should still yield an RVF disadvantage, because of stronger vertical interactions inherent to the RVF/LH. Under my account, vertically-surrounding choices should not have a stronger effect on the RVF letter, because the letter is now read out early enough that is processed prior to the appearance of the choices. In a subsequent investigation (Tydgat and Grainger, submitted), the authors performed experiments with just such conditions. There was no disadvantage for the RVF in these experiments, contradicting their account.

In fact there's evidence that vertically surrounding letters actually have a stronger effect in the LVF/RH than the RVF/LH, the direct opposite of their account. For example consider identification of vertical trigrams presented to a single VF. It's been established that perception of individual letters is equivalent in the two VFs, so any differences between VFs in trigram identification come from interactions amongst the letters. If there are stronger vertical interactions in the RVF/LH, the middle letter of a vertical consonant trigram should be perceived worse in the RVF than the LVF. However, the opposite is the case; the middle letter is perceived better in the RVF than the LVF (e.g., Luh and Wagner, 1997, Brain and Language).

By the way, I have previously addressed the issue of different perceptual patterns for vertical LVF and RVF trigrams, and suggested that trigrams are first mentally transformed to the horizontal (Whitney, 2001, PB&R). I now think it is more likely that the different patterns come directly from vertical interactions. I've proposed that there's strong left-to-right inhibition in the LVF/RH to invert the acuity gradient. This inhibition occurs, by definition, along the horizontal axis. However, in the brain, angular directions are usually coarsely coded, and so it may be the case that the left-to-right inhibition "bleeds" onto the vertical dimension, giving increased vertical interactions in the LVF/RH for letter strings.

Also in opposition to Tydgat and Grainger's account of the final-letter advantage is its dependency on exposure duration. According to the SERIOL model, the final-letter advantage arises when the final letter can fire for a longer period of time than the previous letter. For very brief presentations, there should be no final-letter advantage, even when there are no surrounding letter choices. This is indeed the case. For example Gomez, Ratcliff & Perea (in press, Psych Rev) presented five-letter strings for 60 ms, followed by a mask and 2AFC of two strings presented well below the stimulus location. There was no final-letter advantage at this exposure duration.

In motivating their proposal of hemisphere-specific receptive field shapes, Tydgat and Grainger suggest that the importance of the initial letter causes elongated receptive fields to develop in the LVF/RH and that these fields are asymmetric in that they encompass the letter to the right, but not the left. Note that this implies that LVF letters are quite sensitive to interference from a letter to the left, something that I've been arguing for years. However, I suggest that this is a result of lateral inhibition, rather than receptive-field shape. Note that the presence of additional letters to the left (two or three) has a stronger inhibitory effect in the LVF than the RVF (e.g., Wolford & Hollingsworth, 1974, Perception & Psychophysics). This is easily explained under lateral inhibition, but difficult to explain via receptive fields, as implausibly large fields would be required.

Now for the issue of the W-shaped accuracy pattern. In explaining hemifield accuracy patterns ( e.g., Whitney, in press, LC&P), I have assumed that accuracy is primarily governed by activation at the letter level, which is taken to index the probability that the letter is sufficiently activated to be consciously accessible for report. The probability that the activated letter is the actually the correct one is taken to be approximately constant across the string. Recall that the acuity gradient is steep within the fovea, and is shallow in the parafovea. Thus for parafoveal presentation, the difference in acuity across letter positions is relatively small. So it makes sense to assume that the probability that the correct letter is activated is approximately constant, and that activity level is the primary determinant of accuracy. However, in the fovea, acuity differences across string positions are much larger, meaning that the probability that the correct letter is activated varies substantially with string position.

Hence, for accuracy under foveal presentation, availability for report should be weighted by the probability that the correct letter is activated, which would be inversely proportional to feature-level noise. Feature-level noise is determined by acuity, and also by whether the letter receives left-to-right inhibition (under the assumption that such inhibition may be non-uniform and thereby directly introduce noise).

Hence, for five-letter centrally-fixated strings, the probability that the correct letter is activated is proportional to the acuity gradient, with a decrement at the second position, due to LVF/RH left-to-right inhibition. Next we consider the activity pattern at the letter level, which determines probability of accessibility for report. For foveal presentation, the resulting locational gradient would be fairly steep and smooth. As a result, activation of the first four letters would be determined primarily by firing rate. So activation (at the letter level) would decrease across the first four positions, and then rise for the final letter. The figure below illustrates examples of these two probability patterns (correctness and accessibility). At each position, Accuracy (shown in green) is the product of the two probabilities. This yields a symmetric W shape, in line with the experimental data.

Thus a symmetric W-shaped pattern is not inconsistent with serial processing. Serial processing yields the activity pattern in black, which when combined with the effects of feature-level noise (in red), gives the W-shaped pattern. Now if the task instead involved an explicit temporal component, such as reaction time for letter search in a string, we would expect this pattern to be inverted and weighted by the time that it takes to reach the target letter during serial processing. This would yield an asymmetric, upward-sloping M, which is indeed what is observed for the search task (e.g., Pitchford, Ledgeway, & Masterson, 2008, J. of Res. in Reading).

As for Tydgat and Grainger's account of letter vs symbol patterns, note that this explanation depends on the assumption of retinotopic symbol detectors. However, it seems unlikely that we have sufficient experience with symbols to develop location-specific symbol detectors. Instead, I suggest that the letter pattern is a signature of serial processing. An initial-letter advantage arises because the initial letter has the highest activation at the feature level, which causes it to have the highest activation at the letter level. A final-letter advantage arises when the final letter can fire for an extended period of time. Therefore, the final-letter advantage is less robust (than the initial-letter advantage); it is abolished by very brief exposures, or under lateral masking when the final letter is being processed. Symbols do not show this pattern because they are not processed serially; their perceptual pattern depends only on acuity.

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