From his experiments, Helmholtz was able to prove that attentional selection of one object in the visual field does interfere with the identification of other, presumably unattended, objects.
According to Nancy Kanwisher and Paul Downing the neural representations of dissimilar objects in the image smother each other, and attention acts by biasing this competition. For example, the image attributes of the pertinent object are consolidated while those of irrelevant ones are weakened. In another related study, Kastner and his research team tested this hypothesis in human beings. In their experiments,
“the overall neural response from each of these brain areas was lower when four objects were presented simultaneously above and to the right of the central display in the peripheral parts of the subjects visual field than when the same four objects were presented sequentially in the same locations, even though the total amount of retinal stimulation (integrated over time) was identical in the two cases. Kastner and colleagues interpret these results as reflecting an increasing suppressive effect from competitive interactions among the neural representations of different objects.” (Hoffman, 1994)
In the next experiment performed by Kastner and his team, the reduction in the neural reaction to concurrent compared with successive stimuli was much less intense when attention was directed to one of the four secondary stimuli. Based on the findings of this result, it could be argued that attention guards the depiction of the target item from the intrusive effects of nearby stimuli (Kanwisher & Downing, 1998).
All of the experiments discussed above are very lucid in their rationale and significant for the case of driving while speaking. The experimenters
“not only demonstrate the reduction of response for simultaneously presented objects, and the attenuation of that effect by attention, but also quantify these effects separately within each of the cortical areas that make up the early stages of the visual pathway. This work raises the standards of brain-imaging research well above the routine inventories of brain activations that are the standard fare of the field.” (Kanwisher & Downing, 1998)
It could then be concluded that when a number of stimuli are offered at once, the visual system constructs something “less than the sum of its responses to the items when presented alone”. Although some more work is still needed to determine the workings of neural mechanisms and systems, there is no doubt whatsoever over the life hazard posed by speaking in mobile phones while driving an automobile. We (the users of mobile phones) are not “passive recipients of the information that washes over our sensory receptors, but active participants in our own process of perception”. With an understanding of recent developments in cognitive and neural mechanisms of selective attention, safe driving practices are becoming all the more clear cut. It is up to users of this information to act in line with their interests or in opposition to it.
de Fockert, Jan W., Geraint Rees, Christopher D. Frith, and Nilli Lavie. “The Role of Working Memory in Visual Selective Attention., ” Science. 291.5509 (March 2, 2001): 1803.
Hoffman, James E. “Selective Attention in Vision.”, Science. 263.n5154 (March 25, 1994): 1780(2).
Kanwisher, Nancy, and Paul Downing., “Separating the wheat from the chaff. ” Science. 282. n5386 (Oct 2, 1998): 57(2).