## The algebraic model

The Model Consider an experiment in which the subject has to detect an unpatterned peripheral light flash and respond by pressing a button with one hand at a time. There are four stimulus visual hemifield X response hand conditions, and we can approximate the reaction time in each by partitioning it into schematized sequences of component mental operations, from visual registration V through stimulus detection D to motor programming MP and motor response M, including callosal relay CR. Let Vl denote speed of visual registration in the LH, DL denote the speed of target detection in the LH, MPr denote the speed of motor programming in the RH, Mr denote speed of motor output in the RH, CRlr denote speed of callosal relay from the LH to the RH, and so on. When a target visual field is paired with response by the contralateral hand, then callosal relay is necessary at some stage of the Figure 2. Alternative stages of interhemispheric relay in the SRT paradigm of Poffenberger. LH = left hemisphere, RH = right hemisphere, Vl = visual processing in the left hemisphere, Vr = visual processing in the right hemisphere, Dl = decision in the LH, Dr = decision in the RH, MPl = motor programming in the LH, MPr = motor programming in the RH, Ml = motor response in the LH, Mr = motor response in the RH.

Figure 2. Alternative stages of interhemispheric relay in the SRT paradigm of Poffenberger. LH = left hemisphere, RH = right hemisphere, Vl = visual processing in the left hemisphere, Vr = visual processing in the right hemisphere, Dl = decision in the LH, Dr = decision in the RH, MPl = motor programming in the LH, MPr = motor programming in the RH, Ml = motor response in the LH, Mr = motor response in the RH.

information-processing sequence. Suppose relay occurs at an early visual stage, before stimulus detection, and that the decision, motor programming, and response all occur in the other hemisphere (visual relay; see Figure 2). Then the four stimulus visual field X response hand conditions yield the following:

We can incorporate D, MP, and M into one hemispheric processing component P and get the following: