MODELO RACI

the bilateral incongruent stimulus conditions in Experiment 15. Percentages are plotted as a

function o f the direction o f eye deviation for both a deviated left and a deviated right eye.

BILATERAL STIMULUS CONDITIONS

D EVIATED EYE EYE DEVIATION % RESPONSES

% LEFT ; % STRAIGHT % RIGHT

LEFT V ISU A L FIELD TEMPORAL 73.75 24.12 i

! 1

2.13

N A SA L 1.63 16.5 81.88

RIGHT V ISU A L FIELD TEMPORAL 2.63

' 1 ! 46.88 I 1 1 50.5 N A SA L 64 i ! 1 32.88 3.13 NO N E NO N E 10.89 1 1 82.86 i ! i 6.25

T ab le 6.3. Table o f the mean percentages o f subjects’ responses for all the bilateral stimulus

conditions in Experiment 15. In bold are the percentages o f straight responses on w hich the

statistical analysis were made.

A mixed ANOVA was performed on the percentage of straight responses for incongruent stimuli with experiment (i.e. Exp. 14 vs. Exp. 15) as the between- subject factor and eye deviated (i.e. left visual field eye or right visual field eye) plus nasal/temporal deviation as within-subjects factors. This showed a significant (F(l,14)=34.22, p<.001) main effect of experiment, resulting from a significant decrease in the overall percentage of straight responses between Exp. 14 and Exp. 15 (i.e. 76.67% vs. 30.09%, respectively) and a significant main effect of eye deviated (F(l,14)=15.63, p<.01). This was due to a decrease in the percentage of straight responses when the left visual field eye was deviated and the right visual field straight, compared to when the right visual field eye was deviated and the left visual field straight (i.e. 45% vs. 62%, respectively), confirming, also across experiments, the existence o f a LVF bias which did not interact with experiment. The main factor of nasal/temporal deviation did not reach significance (F<1), but the interaction between

experiment and nasal/temporal deviation was marginal (F(l,14)=3.88, p=.07), showing in Exp. 15 a slightly stronger influence of nasally compared to temporally deviated eyes on the percentage of straight response (i.e. 25% vs. 35%, respectively). As discuss earlier, this was probably due to the use of faster displays in Exp. 15 which might have made just the white part of the eye (i.e. sclera, which was larger in the nasally deviated eye than in the temporally deviated eye or straight eye, see Fig 6.2) a more salient cue forjudging gaze direction. Once again this finding suggests a visual field effect on the perception of gaze.

General discussion

These two experiments found a visual field effect for gaze perception, for the first time. Interestingly, the observed LVF advantage is consistent with that previously found in several face tasks (e.g. Gilbert and Bakan, 1973; Christman and Hackworth, 1993; Luh et al., 1991) and usually attributed to right-hemisphere specialisation for face processing. However, the present effect cannot have been due to the processing of the whole face, since only the regions immediately around the eye were used as the gaze stimuli, neither could it have been due to an eye movement by the subjects, given that the same effect was replicated even for very brief displays (Experiment 15).

There are at least two ways this effect can be interpreted. On one hand, it might be that some of the previous findings regarding apparent LVF dominance in face perception (Gilbert & Bakan, 1973; Luh et al., 1991;

Christman et al, 1993; Kowner, 1995) might be due to the LVF dominance found here for eye perception, in principle just the eye region of whole-face stimuli might have triggered the effect. This seems particularly relevant for LVF advantages in emotional expression tasks, given the importance o f eyes in such expressions (e.g. Argyle & Cook, 1976; Baron-Cohen et al., 1997). However, the LVF dominance found here might equally be part of a bias in face perception generally (even though whole faces were not shown), as eyes are face components. Gaze as part of the face may or may not be subserved by the same system as for other face-parts, although some evidence has already been brought in favour of distinct brain mechanisms for face and gaze processing as shown by recent imaging data (fMRI).

For instance. Puce et al. (1997b) found that, within the occipitotemporal cortex, changes in gaze direction activate areas different from those described for face recognition processing. Hoffman and Haxby;^2000) outlined distinct neuronal networks forjudging eye gaze and face identity, albeit both largely general within the neural system underpinning face perception. Furthermore, evidence of a distributed brain network involving the occipital part of the fusiform gyrus, the right parietal lobule, the right inferior temporal gyrus and bilateral middle temporal gyrus has also been reported by Wicker et al. (1998), specifically involved in the perception of eyes.

Whether or not the present left visual field dominance is due to a process specific to just the eyes alone, or rather to part-based processing of any face components (Clark et al. 1996; Bentin et al. 1996; Eimer, 1998), the present

finding of a LVF advantage for gaze perception still brings new behavioural evidence for a possible right-hemisphere specialisation in gaze perception. It also suggests that accounts which attribute all LVF advantages to holistic processing of the entire face (e.g. Tanaka & Farah, 1993) are incomplete.