The aim of my thesis was to study the visuomotor processes underlying competing responses in a conflict task. According to the "stimulus-response compatibility effect", reaction times (RT) to stimuli placed on the same side are faster than to stimuli spatially opposite to the response. This effect is evident also when stimulus side has to be ignored, as in the Simon task. Indeed, the Simon effect refers to the advantage in responding to spatially compatible stimuli, with respect to incompatible ones. After a brief description of anatomical and functional correlates of perception and motor responses, I review the main theories and models on perception and action (Chapter 1), with a particular emphasis on their interaction during conflict tasks. Then I present five different experiments and discuss them along two chapters. The first two experiments propose an analysis of the time-course of the conflict at response stage. First, I tested whether the conflict between competing responses is solved before movement execution (Chapter 2). For this purpose, I analysed the motor evoked potentials (MEP) elicited by different stimuli, from geometrical shapes to directional symbols (arrows). Results show that RT to arrows are faster than to geometrical shapes, both in compatible and incompatible trials. In contrast to RT differences, MEP relative to arrows and to geometrical shapes are similar both in time and shape. Moreover, a negative activity ipsilateral to the responding hand is present at frontal sites before the onset of the response. Remarkably, this ipsilateral frontal activity is present in incompatible trials only, and it can reflect the EEG correlate of the conflict. Indeed, sources reconstruction seems to support this speculation. Dipoles relative to this ipsilateral frontal activity are located in an area belonging to the superior frontal gyrus (GFs), namely in the dorsolateral superior frontal gyrus (DLSFg), which is a region commonly associated with the selection of the task-relevant response in conflict situations. Thus, RT to arrows result to be faster than to geometrical shapes, whereas MEP waveform and topography are unaffected by the kind of stimuli used. I asked whether directional attributes are encoded at early perceptual stages, rather than at response stage. In a third experiment (Chapter 3), I analysed the visual evoked potentials (VEP) relative to arrows and to geometrical shapes. A N1 component of the VEP comes out to be selectively modulated by arrows but to be insensitive to non-directional shapes. I interpret these data as the confirm that arrows speed up RT because they are directly "translatable" into right-left responding hand, whereas geometrical shapes cannot elicit such a process. This translation takes place at early visual stages. Indeed, N1 source activity is localised in the parietal lobe, a region belonging to the dorsal stream. Thus, directional attributes are encoded by those dorsal regions that will transform spatial properties into the subsequent related actions. In a fourth experiment (Chapter 3), I aim at detailing the neural substrates of the visuomotor processes previously described in their temporal aspects. The Simon-like task with arrows is replicated, but using a fMRI approach. In particular the core of the fourth experiment was to individuate the neural substrates relative to the frontal ipsilateral activity prior to response and to the visual evoked N1 component elicited by arrows. Confirming EEG results, fMRI analysis show activation both in the inferior parietal (IP) lobe contralateral to the stimulus side and, in incompatible trials only, in the ipsi superior frontal gyrus (GFs). The IP activation is considered to match the parietal N1 visual evoked activity. Indeed, as above mentioned, IP is a dorsal region that functionally subserve the translation of spatial inputs into motor coordinates, that is the translation from arrows direction to right-left responses. On the other hand, GFs is selectively activated in incompatible trials only, and it presumably match the pre-movement activity found at ipsilateral frontal electrodes. In a fifth experiment I asked whether arrows represent the best cue to speed up responses. Since arrows indicate direction in a symbolic way, I suggest that gaze, which possess an intrinsic biological relevance, can selectively speed up responses as compared to arrows. Contrary to what expected, responses to gaze are slower than to arrows. It is hypothesised that gaze direction requires a time-consuming translation from one frame of reference (the position of the pupils into the eyes) to another (the lateral position of responses), whereas arrows are directly translated into right-left co-ordinates. An analysis of VEP responses reveals that gaze direction selectively enhances P1 component, with a source in the middle occipital cortex. I concluded that, differently from the dorsal N1 activation induced by arrows, the P1 gaze-triggered modulation reasonably reflects the activation of a ventral, occipito-temporal pathway. In conclusion, the main results of my thesis are as follows. When there is a conflict between competing responses to visual stimuli, this conflict is temporally solved before response execution. The solution of the conflict occurs as an active inhibition of the incorrect response. If visual stimuli are also provided with directional information, these are encoded at very early visual stages. This precocious encoding allows responses to directional stimuli to be faster also in conflicting conditions.
|Autori interni:||Carriero, Lucia|
|Titolo:||To do or not to do: visuomotor processes underlying response conflict|
|Data di pubblicazione:||28-giu-2005|
|Appare nelle tipologie:||8.1 PhD thesis|