The Peripheral Detection Task and Related Methods: Theoretical and Methodological Issues Johan Engström, Driver Metrics Workshop, San Antonio, TX 2008-06-02
Outline The PDT and its variants Theoretical basis: What do we measure with the PDT? Methodological issues TDT and VDT development in AIDE and SafeTE Summary and conclusions Generally based on: Victor, T. Engström, J. and Harbluk, J. (in press). Distraction Assessment Methods Based on Visual Behaviour and Event Detection. In Regan, M., Lee, J. and Young, K. (Eds) Driver Distraction: Theory, Effects and Mitigation. CRC Press. 2 6/16/08
1. The PDT and its variants 3 6/16/08
The Peripheral Detection Task Originally developed by van Winsum, Martens and Herland (1999) Visual stimuli presented peripherally at an interval between 3 and 5 seconds Also some spatial variation The subject responds by means of button attached to the index finger Performance quantified in terms of hit rate and mean response time 4 6/16/08
Variants of the PDT: Alternative stimulus presentation methods PDT with head mounted visual stimulus (Schindhelm et al., 2003; invented by TNO) The Visual Detection Task (VDT): Single, centrally placed visual stimulus: (VDT; Engström and Mårdh, 2007) The Tactile Detection Task (TDT): Tactile stimulus: Engström et al., 2005; Merat and Jamson, 2007) 5 6/16/08
Application to In-vehicle Information System (IVIS) Asssessment Assessing driving demand Driving + PDT compared for different driving conditions Assessing secondary task (e.g. IVIS) demand In static conditions (no driving) PDT (baseline) compared to PDT + IVIS In dynamic conditions (driving) Driving + PDT (baseline) compared to Driving+PDT+IVIS Main focus here 6 6/16/08
(Selective) PDT history IN-ARTE simulator version (van Winsum et al., 1999): Field version (Olsson and Burns, 2000) Swedish and German field studies (IHRA cooperation) (Ekström and Fagerström, 2001; Harms and Patten, 2001; Jahn et al., 2005) The Swedish mobile phone study (Patten et al., 2003) COMUNICAR (Schindhelm et al., 2003) HASTE (Johansson et al, 2006) Original TDT study (Engström et al., 2005) AIDE TDT (e.g. Merat et al., 2006; Mattes et al., 2006; Merat and Jamson, 2007) SafeTE VDT (Engström and Mårdh, 2007) OPTIVE - VDT (ongoing) US developments... (GM, CAMP) 7 6/16/08
2. Theoretical basis of the PDT 8 6/16/08
Why Peripheral? The Tunnel Vision Hypothesis Original motivation (van Winsum et al., 1999) Hypothesis: Workload leads to visual tunneling degrades detection performance more in the peripheral- than in the central field of view (interaction between workload and eccentricity) A simple detection task with peripherally placed stimuli would be sensitive to workload but not affect the primary task Tunnel Vision vs. General Interference 9 6/16/08
Please note: A reduction of the functional field of view due to workload does not imply visual tunneling! Visual tunneling is not the same as gaze concentration! 10 6/16/08
Evidence (cited) for visual tunneling Miura (e.g. 1986) Field studies measuring driving demand with a PDT-like task Found functional field reduction but did not test for Tunnel Vision! Williams (e.g. 1988) Tachistoscope studies with fovealling loading tasks Found visual tunneling, but only in certain conditions (at small eccentricities, instruction to strongly prioritise foveal task) Most of Williams results support General Interference 11 6/16/08
Evidence for general interference van Winsum et al. (1999; Original PDT study) PDT used for driving demand assessment Found no evidence for visual tunneling Found no main effect of stimulus eccentricity at all! Recarte and Nunes (2003) Detection task (similar to PDT) used for secondary (cognitive task) assessment No evidence for visual tunneling In general, no PDT-related study has demonstrated visual tunneling Rather, evidence supports general interference ( cognitive tunneling ) 12 6/16/08
Conclusions so far The sensitivity of the PDT to driving and secondary task demand does not depend on stimuli position (even the main effect of eccentricity seems to be minor) No reason to put stimuli in the periphery > rationale for the VDT Further question: Is the sensitivity of the PDT even independent on stimulus modality? 13 6/16/08
Comparison between different stimulus modalities: Engström et al. (2005); Merat and Jamson, 2007) Compared visual tactile and auditory stimulus presentation (PDT, TDT, ADT) Two secondary tasks Count backwards with 7 (cognitive) Phone dialling (visualmanual + cognitive) Found no difference in sensitivity between modalities General interference even between modalities! Merat and Jamson (2007): AIDE study DT sensitivity (task RT /baseline RT) 14 6/16/08
So, what does the PDT actually measure? PDT sensitivity is due to general (cognitive) interference rather than modality-specific perceptual narrowing For visual stimuli (PDT and VDT) gaze direction may still have some influence (due to the eccentricity effect), but evidence suggest that this effect is minor compared to the cognitive effect PDT and related methods (especially TDT) provides a rather pure measure of cognitive workload This is in contrast to most other workload and driving performance measures (e.g. LCT) which represent the combined effect of cognitive workload load and visual allocation 15 6/16/08
Possible explanations Multiple resource theory Would predict stronger effect for driving + visual task + PDT than e.g. driving + visual task + TDT Inconsistent with empirical results... Alternative explanation: Lack of top-down bias for attention selection Biased competition hypothesis (e.g. Desimone and Duncan, 1985): Attention selection is governed by neural competition driven partly bottom-up by stimulus properties and partly top-down by goals and expectancies PDT stimuli strongly expected gain from top-down bias in baseline conditions During high cognitive load, the top-down bias is cut off Remains to be tested... Cognitive load 16 6/16/08
3. Methodological issues 17 6/16/08
Methodological issues (1) The problem with two performance metrics RT and hit rate not independent -> difficult to interpret... Hit rate yields strongly non-normally distributed data -> cannot use parametric statistics Does PDT affect driving performance? No effects found by Merat and Jamson (2005) For the TDT, some interaction with secondary task difficulty for SDLP and SWRR in demanding driving conditions (Mattes et al., 2005), but no main effects Does PDT affect visual behaviour? Yes, peripheral stimuli are frequently gazed at (Miura, 1986; Engström and Mårdh, 2007) For the TDT, Mattes et al. (2007) found some interactions with secondary task difficulty, but no main effects 18 6/16/08
Methodological issues (2) Learning curve Not thoroughly investigated Instructions/strategies Instructions normally to do perform the PDT as accurately as possible without compromising traffic safety However, effect of instructions not thoroughly investigated Task allocation strategies anecdotically reported (like for LCT) Sensitivity to lighting conditions Often reported problem for the PDT in field trials Less of a problem for VDT and no problem at all for the TDT 19 6/16/08
4. Development of TDT and VDT in AIDE and SafeTE 20 6/16/08
AIDE and SafeTE AIDE EU funded project (PDT development small part) Performed a number of experiments on PDT, VDT, TDT and AD Recommended TDT with VDT as a complement SafeTE Swedish national project ordered by SRA (conducted by Volvo and VTI) Developed a detailed IVIS evaluation methodology VDT and eye-movement-based visual demand measures the basis for the objective part of the assessment 21 6/16/08
Advantages of VDT and TDT compared to the classical PDT VDT Higher conspicuity More valid reponses -> could use RT as single performance metric Less sensitive to lighting conditions Less effect on eye movements TDT Eliminates any retinal eccentricity effect -> pure measure of cognitive workload No major effects on simultaneous measurement of eye movements Insensitive to lighting conditions 22 6/16/08
AIDE TDT validation (Mattes et al., 2007) Simulator study at Daimler Secondary tasks: Visual-manual: SuRT (2 levels) Cognitive: Counting up with 2 (easy) or down with 7 (hard) General results: Separated all task levels except visual easy-hard Relatively robust in different driving conditions The TDT sometimes interacted with IVIS difficulty for visual demand and driving performance measures 23 6/16/08
AIDE TDT and VDT specification (D2.2.3) 24 6/16/08
The SafeTE specification Similar level of detail as an ISO standard, including: Subject characteristics Method for task selection Measurement system set-up Requirements for pilot testing Instructions Familiarisation and training Task annotation Methods for data quality check Visual and cognitive demand calculation Translation to 5-point rating scale 25 6/16/08
5. Summary and Conclusions 26 6/16/08
Summary: Answers to discussion questions (1) 1) Strengths and weaknesses Strengths: Cheap and simple to use Can measure workload peaks High reliability Diagnostic provides a pure measure of cognitive load (cf. LCT) Weaknesses Problem with two performance metrics Affects other measures (?) Task allocation strategies (?) Learning effects (?) Still somewhat unclear theoretical basis (but this is the case for most driver metrics...) 2) For what driver tasks is the metric applicable or not applicable? Applicable to cognitively demanding tasks (with or without visual component) 27 6/16/08
Summary: Answers to discussion questions (2) 3) How can the metric be improved? Mainly by resolving the open issues mentioned above... 4) How does your preferred metric correlate to real world driving on road or test track and to crashes or crash risk? The PDT functions works both in the lab and in the field Relation to crash risk difficult question: Is it possible to extrapolate RTs to (expected) PDT stimuli to RTs to (unexpected) real world critical events. Is the effect of expectancy additive? Probably not but remains to be tested... 28 6/16/08
Summary: Answers to discussion questions (3) 5) What criterion value(s) do you recommend for your preferred metric? Are criteria needed? Depends on purpose... Absolute RT values depend on specific driving situation and/or simulator set-up Calibration tasks (cognitive) probably needed 6) What process did you use (or recommend) to establish a criterion value(s) for your selected performance metric(s)? What is your rationale? Don t have any... Rating scale could be established by means of systematic testing using calibration tasks 29 6/16/08
Summary: Answers to discussion questions (4) 7) Is the metric/method ready for standardization? If not, what research is needed to enable standardization? In general, yes especially TDT and VDT have proven both sensitive and reliable We need a standard method for assessing cognitive load (as complement to occlusion and eye movement measurement standards) The AIDE and SafeTE specifications provide good starting points Further work needed to resolve the open issues, in particular: 30 6/16/08
General conclusions PDT and related methods provide a pure measure of cognitive workload (or interference with central attention selection mechanisms) Good complement to visual behaviour measures TDT, VDT (and possibly head-mounted PDT) recommended for further consideration A number of theoretical and methodological issues need to be addressed before it can be standardised 31 6/16/08
Key elements of a potential standard General measurement system set-up Detailed specifications for stimulus presentation and response method Experimental design Subject instructions Procedures for familiarisation and training Task annotation method Methods for data quality check Data analysis 32 6/16/08
Backup slides 33 6/16/08