Target validation-invalidation

Figure 3. The covert orientation of attention paradigm, a) Subjects visually fixate on the central cross throughout. A cue (in this case a central arrow), which may be valid, invalid or neutral directs attention to the correct, incorrect or neither target location respectively, b) The typical cost and benefit of valid and invalid cueing.

Patients with schizophrenia perform poorly on tasks requiring cognitive control and contextual information processing (Stroop, AX-CPT, go/no-go). In the typical AX-CPT task (Rosvold et al., 1956), participants are shown a series of letters, presented one at a time. The letter X is designated as a target, but only when it is immediately preceded by a valid cue (letter A ) X preceded by an invalid cue (a letter other than A BX trials), and other letters preceded by A (AY trials) or any other letter (BY trials) are... 

The equivalent-local form of the coupled-channels-optical method does not give a satisfactory description of the excitation of triplet states (Brun-ger et al, 1990). Here only the exchange part of the polarisation potential contributes. The equivalent-local approximation to this is not sufficiently accurate. It is necessary to check the overall validity of the treatment of the complete target space by comparing calculated total cross sections with experiment. This is done in table 8.8. The experiments of Nickel et al. (1985) were done by a beam-transmission technique (section 2.1.3). The calculation overestimates total cross sections by about 20%, due to an overestimate of the total ionisation cross section. However, an error of this magnitude in the (second-order) polarisation potential does not invalidate the coupled-channels-optical calculation for low-lying discrete channels. 

---