VISUAL ATTENTION 197 198 DESIMONE DUNCAN of objects over retinal translation (Gross&Mishkin 1977.Lueschow et al the competition in their favor.This issue,which we term selectivity,is con- 1994. sidered in later sections. These receptive fieldscn be viewed asacritical visual proce ing re source. If the dorsal stream receives its visual input in parallel to the ventral stream for which objects in the visual field must compete(Desimone 1992.Olshausen as the anatomy suggests (Desimone Ungerleider 1989),then it is presumably et al 1993.Tsotsos 1990).If one were to add ever more independent objects faced with competition among objects as well.As in IT cortex,receptive fields to a V4 or IT receptive field,the information available about any one of them in posterior parietal cortex are very large,and it seems likely that increasing would certainly decrease.If,for example,a color-sensitive IT neuron were to the number of independent objects in the visual field will ev integrate wavelength over its large receptive field,one might not be able to the c parietal cor to ext act the locations of eah of them in tell from that cell alone if a given level of response was due to,say,one red object or two yellow ones or three green ones at different locations in the field. etition.to the must also deal nt th dist y filter of the Such ambiguity may be responsible for the interference effects found in divided input (e.g. &Wurtz 1993a,b.U1 fo attention sible eyes to only arget at a time.Acritic e is how may be reduced.in part by linking objects and their features selectivity is coord different systems so that the same target retinal o ed that object is selected for perceptual and spatial analysis as well as for motor tfro om the entra stbe supp cd by th control. e In fact,the ven about the location of complex object featur es.V and TEO eurons process relatively sophisticated information about SELECTIVITY:SCREENING OUT UNWANTED STIMULI object shape (Desimone Schein 1987,Gallant et al 1993.Tanaka et al 1991)and have retinotopically organized receptive fields(Boussaoud et al 1991.Gattass et al 1988).At any Behavioral Data given retinotopic locus in these areas,receptive fields show considerable The ability to screen out irrelevant objects(Figure 1)is not absolute.It is easy scatter.One could.in principle.derive information about the relative locations in some cases and difficult in others.as is well illustrated in visual search.The of nearby features from a population of cells with partially overlapping fields subject detects or identifies a single target presented in an array of nontargets the same way one could derive information about a specific color from a Examples are shown in Figure 3.In easy cases,the target appears to"pop out" population of neurons with broad but different color tuning.Similarly,although of the array,as if attention were drawn directly to it (Donderi Zelnicker receptive fields in IT cortex may span 20-30 degrees or more,they are not 1969,Treisman Gelade 1980).Under such circumstances,the number of ogeneous Typically.the fields have a hol spot nontargets has little effect on the speed or accuracy of target detection or may extend asyr etrically into the or lo wer contralateral visual field. identification.In hard cases,how are not filtered out well.In Alth gh the ces o of IT n the sam these instand number of nta edisplay has a large effect on ions,for ge min ab for eac anges signi i.e cells are t Gelade 1980) hough in fact me w y are tuned to oth er obj ect features (Desi one et a ■ b) Thus,in principle,objects and their locations might be linked to some exten Q within the ventral stream.Even so,parallel processing across the visual field 0 is likely to be limited To sum up.retinal location,as with other object features,is coarsely coded in the ventral stream.Information about more than one object may,to some Q ■ X extent,be processed in parallel,but the information available about any given object will decline as more and more objects are added to receptive fields n the target is a mis atching Therefore,objects must compete for processing in the ventral stream,and the visual svstem should use anv information it has about relevant ohiects to hias VISUAL ATTENTION 197 of objects over retinal translation (Gross & Mishkin 1977, Lueschow et al 1994). These receptive fields can be viewed as a critical visual processing resource, for which objects in the visual field must compete (Desimone 1992, Olshausen et al 1993, Tsotsos 1990). If one were to add ever more independent objects to a V4 or IT receptive field, the information available about any one of them would certainly decrease. If, for example, a color-sensitive IT neuron were to integrate wavelength over its large receptive field, one might not be able to tell from that cell alone if a given level of response was due to, say, one red object or two yellow ones or three green ones at different locations in the field. Such ambiguity may be responsible for the interference effects found in divided attention. This ambiguity may be reduced, in part, by linking objects and their features to retinal locations. It is sometimes presumed that location information is absent from the ventral "what" stream altogether and must be supplied by the dorsal "where" stream. In fact, the ventral stream itself contains information about the retinal location of complex object features. V4 and TEO neurons process relatively sophisticated information about object shape (Desimone Schein 1987, Gallant et al 1993, Tanaka et al 1991) and have retinotopically organized receptive fields (Boussaoud et al 1991, Gattass et al 1988). At any given retinotopic locus in these areas, receptive fields show considerable scatter. One could, in principle, derive information about the relative locations of nearby features from a population of cells with partially overlapping fields the same way one could derive information about a specific color from a population of neurons with broad but different color tuning. Similarly, although receptive fields in IT cortex may span 20-30 degrees or more, they are not homogeneous. Typically, the fields have a hot spot at the center of gaze, which may extend asymmetrically into the upper or lower contralateral visual field. Although the stimulus preferences of IT neurons remain the same over large retinal regions, for a large minority of cells the absolute response to a given stimulus changes significantly with retinal location, i.e. cells are tuned to retinal location the same way they are tuned to other object features (Desimone et al 1984, Lueschow et al 1994, Schwartz et al 1983; also see Chelazzi et al 1993a). Thus, in principle, objects and their locations might be linked to some extent within the ventral stream. Even so, parallel processing across the visual field is likely to be limited. To sum up, retinal location, as with other object features, is coarsely coded in the ventral stream. Information about more than one object may, to some extent, be processed in parallel, but the information available about any given object will decline as more and more objects are added to receptive fields. Therefore, objects must compete for processing in the ventral stream, and the visual system should use any information it has about relevant objects to bias www.annualreviews.org/aronline Annual Reviews Annu. Rev. Neurosci. 1995.18:193-222. Downloaded from arjournals.annualreviews.org by University of California - San Diego on 01/05/07. For personal use only. 198 DESIMONE & DUNCAN the competition in their favor. This issue~ which we term selectivity, is con￾sidered in later sections. If the dorsal stream receives its visual input in parallel to the ventral stream as the anatomy suggests (Desimone & Ungerleider 1989), then it is presumably faced with competition among objects as well. As in IT cortex, receptive fields in posterior parietal cortex are very large, and it seems likely that increasing the number of independent objects in the visual field will eventually exceed the capacity of parietal cortex to extract the locations of each of them in parallel. Likewise, neural systems for visuomotor control must also deal with competition, to the extent that distractors are not already filtered out of the visual input (e.g. Munoz & Wurtz 1993a,b). Ultimately, for example, it possible to move the eyes to only one target at a time. A critical issue is how selectivity is coordinated across the different systems so that the same target object is selected for perceptual and spatial analysis as well as for motor control. SELECTIVITY: SCREENING OUT UNWANTED STIMULI Behavioral Data The ability to screen out irrelevant objects (Figure 1) is not absolute. It is easy in some cases and difficult in others, as is well illustrated in visual search. The subject detects or identifies a single target presented in an array of nontargets. Examples are shown in Figure 3. In easy cases, the target appears to "pop out" of the array, as if attention were drawn directly to it (Donderi & Zelnicker 1969, Treisman & Gelade 1980). Under such circumstances, the number of nontargets has little effect on the speed or accuracy of target detection or identification. In hard cases, however, nontargets are not filtered out well. In these instances, the number of nontargets in the display has a large effect on performance. An increase of 50 ms in target detection time for each nontarget added to the array is typical (Treisman & Gelade 1980), though in fact, this ¯ ¯ P [] [] C ¯ X 3 J Figure 3Selectivity in visual search. Target pop-out is revealed when the target is a mismatching element in an otherwise homogeneous field (panel a). Search is also extremely easy, however, whenever targets and nontargets are highly discriminable. Pop-out can also be based on more complex properties (panel b; search for the single digit). www.annualreviews.org/aronline Annual Reviews Annu. Rev. Neurosci. 1995.18:193-222. Downloaded from arjournals.annualreviews.org by University of California - San Diego on 01/05/07. For personal use only