Estudios de expertos
V. MAYORÍA DE POBLACIÓN VACUNADA
In many respects the model developed by Di Lollo et al. (2000) is similar to Turvey’s in that it is assumed that the input stimulus activates many different sorts of processing modules akin to those shown in Figure 4.3. These in turn feed forward information to more cen- tral mechanisms. Although many different processing modules are posited across the whole of the retinal mosaic, let us consider the most simple case and focus on just one item location in the stimulus display – on just the operation of one processing module. The input
layer is defined in terms of a retinal (point-for-point)
representation of the display item (it is a retinotopic map – see Chapter 3). Information from the input layer feeds directly into the working space layer that codes information in a similar point-for-point manner. However, at each stage in processing, comparisons are made between the input layer and working space layer representations (in the same way that an artist
Figure 4.3 Schematic representation of the re-entrant model of processing set out by Di Lollo et al. (2000)
The input layer contains a record of the input stimulus and this is fed forward to the working space immediately following the onset of the stimulus. The working space representation, in turn, feeds forward to the pattern space. The pattern space contains stored pattern knowledge such as what an ‘A’ looks like. If the working space representation is consistent with a stored pattern then information from the pattern layer is fed back onto the working layer. Such feedback (i.e., re-entrant processes) means that the working layer representation is over-written by the pattern information. Comparisons between the working space representation and the input (the © in the illustration) are continuously being undertaken and the whole set of processes iterates until a pattern detector in the pattern space reaches some criterion level of activation.
Source: Enns, J. T., & Di Lollo, V. (2000). What’s new in visual
masking? Trends in Cognitive Science, 4, 345 –352 (fig. 5, p. 348). Reproduced with permission from Elsevier.
continues to compare their drawing with real life while making a sketch), such that the eventual working space layer representation captures the combination of these two. At the start of processing the working space will be empty (an artist’s blank page) and when a stimulus is presented its input representation is merely copied into the working space. There is nothing to combine with the input representation.
Pattern information is represented at the final
pattern layer where shape characteristics are made
explicit. The pattern layer is activated by the working space representation. So in simple terms, if the work- ing space representation is consistent with the pres- ence of the letter ‘A’ then this pattern information will be activated at the level of the pattern layer. Following this, pattern information is fed back onto the working space. What this actually means is that the activated pattern information over-writes the contents of the working space buffer. This sort of feedback defines the re-entrant aspect of the model. Processing then proceeds whereby the input layer and working space layer representations are again compared and a new working space representation is constructed. The time to identify the stimulus is gauged by the time it takes a given pattern detector (such as the letter detector for the letter ‘A’) to reach a pre-defined level of activation.
It is the over-writing processes that are used to explain masking by object substitution. In very simple terms, initially the input layer will contain a composite repres- entation of the target item and the surrounding masker and this is immediately propagated onto the working space layer. This in turn produces activation at the level of the pattern layer which may be ambiguous to the nature of what is present – the retinal input contains information about both target and mask so any feed- back from the pattern layer will also be ambiguous. Next the target is removed but the mask remains and so now all that will be consequently active at the pattern layer is the detector for the mask. This activation will over-write any residual representation of the target in the working space layer. Hence the target will be rendered invisible. In the re-entrant account we have a rather extreme form of feedback being discussed. The present best guess as to the nature of the stimulus is being used to over-write (erase) the current output from stimulus encoding mechanisms. So although Enns and Di Lollo (2000) are keen to distance themselves from simple interruption accounts of masking, the re-entrant account does assume that, at a very basic level, the input representation is erased by the currently present stimulus. This does seem therefore to be fundament- ally an account of masking based on the notion of replacement. ‘See ‘What have we learnt?’, below.
‘What have we learnt?
The present discussion of feedback processes has provided one specific example but, as you will dis- cover, many different sorts of feedback processes have been discussed in the literature. In one respect, the re-entrant processes seem extremely constraining. Consider working in market research. You make an initial appraisal of the evidence in favour of a certain product and send this forward to head office, who then simply over-write your appraisal and state that ‘This is what the evidence is actually telling us.’ Of course, this may actually be a fair appraisal of the processes underlying object substitution. However, other less severe accounts of feedback have been discussed and we will consider these as the material unfolds.
Further discussion of the re-entrant model, how- ever, would be premature in advance of considering of the nature of visual attention. At the level of detail, the account is one based on assumptions about directed visual attention. We will consider
the nature of attention later in the book. However, discussion of the re-entrant model has provided us with our first example of how knowledge of the world may affect our perception of it. The re- entrant processes in the model play an important role in deriving a perceptual representation of the input. In this regard, we are now beginning to ques- tion the whole notion of a strictly sequential stage account of processing because this is contradicted by the model. Following the first initial feedforward of information in which the three layers are activ- ated in series, the re-entrant processes allow the flow of control to operate in both directions. More particularly information from a later level of analy- sis can influence processing at an early stage. Such ideas as these are central to debates about how it is that knowledge may affect perception and ulti- mately how these may inform questions about the nature of conscious perception.
saw an E but how confident are you that you actually saw an E (1 – a pure guess, . . . 10 – absolutely certain). Using such confidence ratings, in tandem with the perceptual report data, we can be more certain about what information has truly been perceived. The central problem is that because consciousness is by definition subjective, we have no objective criteria against which to judge it. We can only go on what the participants (in a sense) tell us. Indeed, we will see just how important concerns about guessing are as we proceed.
The nature of consciousness may seem a very long way from any possible differences between random noise and pattern masking, etc., but it simply is the case that the consideration of the nature of various masking effects has been used to adjudicate between so-called conscious and non-conscious (un-conscious) processing. As a starting point we will consider the work of Allport (1977). Although this study used visual masking and examined the nature of iconic memory, it also provides an example of how visual masking techniques have been used to address some fairly fundamental issues about human cognition.