On the Foundations of Shortcut Learning

Deep-learning models can extract a rich assortment of features from data. Which features a model uses depends not only on predictivity-how reliably a feature indicates train-set labels-but also on availability-how easily the feature can be extracted, or leveraged, from inputs. The literature on shortcut learning has noted examples in which models privilege one feature over another, for example texture over shape and image backgrounds over foreground objects. Here, we test hypotheses about which input properties are more available to a model, and systematically study how predictivity and availability interact to shape models' feature use. We construct a minimal, explicit generative framework for synthesizing classification datasets with two latent features that vary in predictivity and in factors we hypothesize to relate to availability, and quantify a model's shortcut bias-its over-reliance on the shortcut (more available, less predictive) feature at the expense of the core (less available, more predictive) feature. We find that linear models are relatively unbiased, but introducing a single hidden layer with ReLU or Tanh units yields a bias. Our empirical findings are consistent with a theoretical account based on Neural Tangent Kernels. Finally, we study how models used in practice trade off predictivity and availability in naturalistic datasets, discovering availability manipulations which increase models' degree of shortcut bias. Taken together, these findings suggest that the propensity to learn shortcut features is a fundamental characteristic of deep nonlinear architectures warranting systematic study given its role in shaping how models solve tasks

Temporal dynamics of the neural representation of hue and luminance contrast

Hue and luminance contrast are the most basic visual features, emerging in early layers of convolutional neural networks trained to perform object categorization. In human vision, the timing of the neural computations that extract these features, and the extent to which they are determined by the same or separate neural circuits, is unknown. We addressed these questions using multivariate analyses of human brain responses measured with magnetoencephalography. We report four discoveries. First, it was possible to decode hue tolerant to changes in luminance contrast, and luminance contrast tolerant to changes in hue, consistent with the existence of separable neural mechanisms for these features. Second, the decoding time course for luminance contrast peaked 16-24 ms before hue and showed a more prominent secondary peak corresponding to decoding of stimulus cessation. These results are consistent with the idea that the brain uses luminance contrast as an updating signal to separate events within the constant stream of visual information. Third, neural representations of hue generalized to a greater extent across time, providing a neural correlate of the preeminence of hue over luminance contrast in perceptual grouping and memory. Finally, decoding of luminance contrast was more variable across participants for hues associated with daylight (orange and blue) than for anti-daylight (green and pink), suggesting that color-constancy mechanisms reflect individual differences in assumptions about natural lighting

Millennial soil retention of terrestrial organic matter deposited in the Bengal Fan

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 11997, doi:10.1038/s41598-018-30091-8.The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.We acknowledge funding support from the Agouron Institute Postdoctoral Fellowship (K.L.F), the US National Science Foundation (Awards: OCE-1333387 and OCE-13333826), the Investment in Science Fund given primarily by WHOI Trustee and Corporation Members, and the Swiss National Science Foundation (Award: 200020_163162)

The Grizzly, April 21, 1989

Spring Weekend a Whopper! • Ours Nouveau • Sunday\u27s Reception Huge Success • Letter: Shed Miniskirts for Spandex • Heritage Day • Berman Roofing Top Hat Affair • \u27Packers Hike Hick Hills • Ground Round: Super Service • Running\u27s More Than Just Winning • Spring Sports: Hot and Cold • Mr. Ursinus a Whomping Good Time!https://digitalcommons.ursinus.edu/grizzlynews/1236/thumbnail.jp

The Grizzly, May 2, 1989

Greeks Grab Spotlight • Grad. Speakers Tapped • Benedict No.1 • Letter: Racism! Not Just Rednecks • Williams\u27 Farewell • Richter Honored • ISIC\u27s for Going Abroad • The Arena: Seeking Opinions • Trenton Draws First Blood • U.C. Hits Stumbling Block • Positive for MAC\u27s • Crabs and Fries: The Spice of Life • Dance Marathon Coming • Senior Altruism Needed • Seniors\u27 Lasting Impressions: Your Most Memorable Moments at U.C.? • Final Exam Schedulehttps://digitalcommons.ursinus.edu/grizzlynews/1237/thumbnail.jp

The Grizzly, March 3, 1989

Quad Asbestos Cover-Up? • UC Lip-synching Talents Shown at Airband • Letters: Klee Clears Clutter; Fondots Tepid About Water; Students Concerned • Bond is Back! • Isaac\u27s Astounds Aussie • Men\u27s Indoor Ready for Mainstream • Youthful Bears Swim to 3rd • Senior Trio Leads Ursinus • McGowan\u27s Courage Inspiring • Bears End Season Upbeat • Send North Up The River?https://digitalcommons.ursinus.edu/grizzlynews/1231/thumbnail.jp

The Grizzly, February 3, 1989

Hoc\u27sters Hammer Out Honesty Hazards • Middleton Mitts Mega-Bucks to Make Meta-Blood • Absentees Abound: Apathy Apparent • Letter: Bussers and Shirts Ream Reed • Pinsker\u27s Pace Paralyzes Prof. Pack • Grim Groans at Rock Ignorants • Plan B Plants U.C. in Place • Lady Bears Breeze by Lehigh • Hoopsters Haul Ball • AquaBears Swim On • Seniors Snarf Steak • Cinders Casting • Durst Demonstrates Decking Deftness • U.C. Supposedly Safe and Sound • Fruit Fantasy Makes Maxi Delight • U.C. Mourns Losshttps://digitalcommons.ursinus.edu/grizzlynews/1227/thumbnail.jp

The Grizzly, September 28, 1989

Berman Art Center Ready to Roll • New Forum Frontier to be Forged • Letter: Granite Windows Grosses Grad • Stern Happy at Ursinus • Bear Pack Run: Join Us! • Bush Wacked at Drug Dialog • Security New Park Patrol • Wellness Days! • Crutcher Leads Lady Bears to Win • Ursinus Nipped by WMC • Soccer: So-So • V-ball: Optimistic View • Athletes of the Week • Hallinger Competes on Wheels • Calliope Blast from Pasthttps://digitalcommons.ursinus.edu/grizzlynews/1242/thumbnail.jp